Click here for Frequently Asked Questions on Radioactive Iodine and Thyroid Cancer.

General Information

Radioactive iodine (131-I) is a radioactive isotope that is administered orally, in a liquid or capsule form. The majority of radioactive iodine is taken up by your remaining thyroid cells, since the thyroid normally uses iodine to make thyroid hormone. Very small amounts of iodine may also be taken up transiently by cells in your stomach, salivary glands, and ovaries/testicles. It is necessary for patients to either receive injections of Thyrogen or to be withdrawn from thyroid hormone in order for radioactive iodine treatment to be effective. Hypothyroidism results in increased levels of TSH production by the pituitary, which stimulates uptake of the radioactive iodine by thyroid cells.  Recombinant TSH (Thyrogen) is approved for diagnostic radioactive iodine scans and thyroglobulin stimulation, and is also approved in Canada as of August 28 2009 for therapeutic administration of radioactive iodine for remnant ablation in subjects with thyroid cancer who have no evidence for spread of cancer outside the neck.  As Thyrogen is a drug commonly dispensed in the outpatient setting, it is not frequently not covered by hospital pharmacies and patients generally need to obtain insurance coverage for Thyrogen use. Patients may review a Detailed Product Monograph describing Thyrogen use or a more simplified Patient-friendly overview of Thyrogen information.

In some instances, it may be possible to have radioactive iodine administered as outpatient therapy in your own home. To obtain the TGH/UHN guidelines for this outpatient therapy, please review and complete the Outpatient Screening Eligibility Form. If you may be a candidate for this treatment, please review the General Information for Patients Receiving Outpatient Radioactive Iodine. For for more concise general patient information about radioactive iodine, review our simple 131-Iodine Patient fact sheet. Although the preceding document was written primarily for patients receiving inpatient radioactive iodine in the hospital, the general concepts and recommendations are also applicable to patients who are eligible for outpatient therapy.

Before Your Admission or Outpatient Radioactive Iodine Treatment

Guidelines for the use of outpatient doses of radioactive iodine are evolving. For an overview of the considerations relevant to outpatient therapy of radioactive iodine, see Treatment of thyroid carcinoma: emphasis on high-dose (131)i outpatient therapy. J Nucl Med Technol. 2002 Dec;30(4):165-71. Considerable emerging evidence suggests patients with low risk disease may benefit from low doses of radioactive iodine, and in some instances, may not need radioactive iodine at all. See Ablation with low-dose radioiodine and thyrotropin alfa in thyroid cancer N Engl J Med. 2012 May 3;366(18):1674-85. and Strategies of radioiodine ablation in patients with low-risk thyroid cancer N Engl J Med. 2012 May 3;366(18):1663-73

Your thyroid hormone must be stopped prior to your treatment with radioactive iodine treatment.  In some countries, including Canada, recombinant TSH or Thyrogen is approved for use in preparing selected patients for radioactive iodine without the need to stop thyroid hormone. For patients who need to stop thyroid hormone,  L-thyroxine (also commonly known by brand names such as Synthroid, Eltroxin or Levoxyl) should be stopped for at least four and preferably six weeks prior to the radioactive iodine. Patients who have been on very high does of thyroxine for long periods of time may need to stop the thyroxine more than 4-6 weeks prior to their treatment in order for the TSH to increase sufficiently.

Patients taking Cytomel (liothyronine or T3) should stop this medication at least 10 days and preferably two weeks prior to the admission. There does not appear to be any major advantage in going on Cytomel for several weeks versus simply stopping thyroxine for 3-4 weeks, as outlined in L-T(3) preparation for whole-body scintigraphy: a randomized-controlled trial. Clin Endocrinol (Oxf). 2007 Jul 20; [Epub ahead of print]

For women and to a lesser extent men of child bearing age, give some thought, if appropriate, to birth control, as pregnancy should be avoided for the first 6-12 months after radioactive iodine due to the increased risk of miscarriage in women who have been treated with radioactive iodine for thyroid cancer. There is no evidence for an increased risk of miscarriage in the female partners of men who have received radioactive iodine, however in some small studies, a transient decrease in sperm count has been noted in the first year after radioactive iodine. 

Some centers advocate the use of a low iodine diet for one week prior to administration of radioactive iodine to enhance maximal iodine uptake during the radioactive iodine treatment. The rationale underlying implementation of a low iodine diet is that more of the exogenous radioactive iodine will be taken up by any remaining thyroid cells if our own bodies are relatively depleted of iodine. These diets may be somewhat difficult to follow due to the abundance of iodine in many foods, and may be particularly challenging for patients, who for a variety of other medical reasons, already follow a specific diet. Periods longer than 1-2 weeka of low iodine diet adherence do not appear to increase the uptake in the thyroid bed Two weeks of a low-iodine diet are equivalent to 3 weeks for lowering urinary iodine and increasing thyroid radioactive iodine uptake Thyroid. 2011 Jan;21(1):61-7. Epub 2010 Dec 16.  For patients who wish to avoid iodine rich foods and Follow a Low Iodine Diet, an overview of Iodine Content in Foods is provided. Information and references dealing with a low iodine diet has also been compiled by Thryvors

No randomized controlled studies exist to demonstrate the long term benefits of such a diet, however the available data suggests that iodine depletion may enhance our ability to take up more iodine if it is administered exogenously during the treatment. A retrospective study of patients who followed either a normal or low iodine diet prior to radioactive iodine administration has been done. Patients who followed a low iodine diet had greater retention of radioactive iodine in the neck, and a modest but significant increase in objective parameters indicative of thyroid ablation, such as levels of thyroglobulin assessed 6 months later after withdrawal of thyroid hormone. Thus, a low iodine diet (no seafood for 1 week, avoid/minimize the other foods listed below for 4 days) may increase the chances of successfully ablating any remaining thyroid tissue, as shown in Effects of low-iodide diet on postsurgical radioiodide ablation therapy in patients with differentiated thyroid carcinoma. Clin Endocrinol (Oxf). 2003 Apr;58(4):428-35.

                                           Summary of the 4-day low-iodide diet as used in the Leiden University Medical Center

Not allowed Allowed
Fish and sea products All fish and sea products*
Bread Commercial bread Bread baked with iodide-free salt
Crackers, biscuits
Salt Iodide-enriched salt, seasalt Iodide-free salt
Water Tapwater Mineral water
Alcoholic beverages All alcoholic beverages Beer (max. 2 glasses daily)
Coffee, tea If made with mineral water
Eggs All eggs and products containing eggs
Dairy products Salted butters and margarines Unsalted butter and margarines
Foreign cheese Dutch cheese (max. 20 g daily)
Milk (max. 1 glass daily)
Meat Game and poultry Pork, beef, veal, horse
All liver products (max. 100 g daily)
Fruits Specified fruits and fruit-juices Specified fruits and fruit-juices
(max. 2 portions daily)
Vegetables Specified fresh vegetables Specified fresh vegetables
Canned or frozen vegetables
Soups and sauces Commercial soups and sauces Home-made soups and sauces
Colourings/dyes E-127 (erythrosine) E-124 (cochenille red A)
Medication, soap, shampoo Specified iodide-containing products

  *Not allowed from 1 week before radioiodide administration.

It is particularly important to avoid ingestion of iodine rich supplements prior to treatment with radioactive iodine, as iodine may be found in some "natural extracts" or kelp-containing preparations.  

Once you have stopped the thyroid hormone you may feel tired, have minor to moderate muscle aches, and perhaps gain a few pounds due to fluid retention attributable to the absence of thyroid hormone and the development of hypothyroidism.  In general, stopping thyroid hormone for a short period of time is well-tolerated by most patients, and many patients, more often younger individuals, have no symptoms during the period of time off thyroid hormone. The majority of patients can work and function normally during this time, although energy and cognitive function (memory, concentration etc) may be slightly impaired in some individuals) and most patients clearly exhibit reduced quality of life parameters when this is formally assessed by objective testing. To ensure that you have achieved the appropriate degree of TSH elevation and hypothyroidism, a TSH and sometimes a thyroglobulin blood test may be done just prior to the treatment with radioactive iodine. 

During Your Admission

Radioactive iodine is usually administered in capsule form.  Most patients have no major side effects after taking the radioactive iodine capsule.  Some tenderness in the neck is common as the radioactive iodine often is taken up by remnants of normal thyroid cells and the salivary glands under the jaw. Chewing gum, or sucking on hard candies intermittently throughout the admission, starting 24 hrs after the administration of radioactive iodine, will help discharge the radioactive iodine from the salivary glands and lessen this inflammation. It is not uncommon for patients to notice some dryness of the mouth or decrease in taste sensations for a few weeks after the radioactive iodine, due to its effect on the salivary glands. For an overview of this issue, see The role of radioactive iodine in salivary gland dysfunction. Ear Nose Throat J. 2000 Jun;79(6):460-8. Review and Scintigraphic Evaluation of Salivary Gland Dysfunction in Patients with Thyroid Cancer After Radioiodine Treatment. Clin Nucl Med. 2002 Nov;27(11):767-771.

Radioactive iodine can also be taken up by cells lining the stomach. Some patients may experience nausea, stomach upset and rarely vomiting, for which relief in the form of medications such as Gravol can be provided. If you are already feeling queasy or nauseated prior to ingesting the radioactive iodine, consider taking a Gravol before the radioactive iodine pill to minimize the chances of vomiting.

Female patients of reproductive age should have a pregnancy test done at the time of admission to make sure they are not pregnant. Patients may not have visitors during the initial period of their hospitalization. Close body contact, or hugging/kissing, or having subsequent visitors eat or drink in the room is strongly discouraged. The radiation levels will be checked to ensure your levels are sufficiently low prior to discharge.

For additional information on long term consequences and general safety of radioactive iodine, see in JAMA 1998 280:347-355 or the study in Lancet 1999 353:2111-2115. or Complications of radioactive iodine treatment of thyroid carcinoma J Natl Compr Canc Netw. 2010 Nov;8(11):1277-86

Post-Radioactive Iodine Treatment

It is important to drink reasonable amounts of fluids, and to make sure your bowel habit is regular at the time of, and several days after the radioactive iodine treatment. The radioactive iodine that is not taken up by thyroid cells must be excreted in the urine and stool before patients can be safely discharged. Accordingly, it is a good idea to make sure that your bowels are moving regularly to enhance elimination of the radioactive iodine from your system.

You may restart taking your thyroid hormone, after receiving instructions from your physician, often  the day after you are discharged from hospital. Most patients will start L-thyroxine, however some patients are started on both L-thyroxine, and a small dose of Cytomel (T3), in which case the T3 is usually discontinued after about 10 days to 2 weeks. Patients with a known history of heart disease or who may be at risk for developing heart disease should consult with their physician before combining T3 and T4, even for short periods of time.

You will be generally be seen for follow up approximately four to eight weeks after your discharge from the hospital, at which time the blood levels for thyroid hormones, TSH, and thyroglobulin will be drawn to make sure you are on an appropriate dose of L-thyroxine to maintain your TSH in the suppressed range. In some centers, a total body scan is done, from several days to 2 weeks following administration of the radioactive iodine. In over 98% of cases, nothing alarming is seen on this scan, and some studies have even questioned the utility of a post-radioactive iodine total body scan for patients with thyroid cancer confined to the thyroid J Clin Endocrinol Metab 2000 Jan;85(1):175-8 Is diagnostic iodine-131 scanning useful after total thyroid ablation for differentiated thyroid cancer?. The vast majority of patients will have some uptake of iodine in the neck region due to a few remaining thyroid cells, and a dark spot in the region of the bladder may also be visualized on the scan. 

It is recommended that patients not become pregnant for the first 6-12 months following the radioactive iodine treatment.  Although there is no evidence of an increased risk of birth defects in women who do become pregnant in this time period, there is a slightly higher rate of miscarriage in the year following radioactive iodine treatment for thyroid cancer. Hence, birth control is recommended for women in the reproductive age group. As small amounts of radioactive iodine may remain in your body for several weeks after the radioactive iodine treatment, breast feeding should also be discontinued in nursing mothers.

As the radioactive iodine may be taken up by the salivary glands, some degree of dryness in the mouth or transient taste impairment may be noted for several weeks to months after the radioactive iodine treatment. Even less commonly, patients may present at a later date with episodes of salivary gland swelling, which is often self limiting and gradually goes away on its own

A summary of this information is contained within a PDF file of the UHN patient information sheet for Radioactive Iodine Administration. The risks and benefits of radioactive iodine are reviewed in The benefits and risks of I-131 therapy in patients with well-differentiated thyroid cancer. Thyroid. 2009 Dec;19(12):1381-91.


Does every one with a diagnosis of thyroid cancer need treatment with radioactive iodine?

The recommendation to treat with radioactive iodine depends on the extent of surgery, size, number (single vs multifocal) and type of tumor, pathology report, and individual circumstances. For an overview, see Post-surgical use of radioiodine (131I) in patients with papillary and follicular thyroid cancer and the issue of remnant ablation: a consensus report. Eur J Endocrinol. 2005 Nov;153(5):651-9. In the past, it was routine to treat nearly all thyroid cancer patients with radioactive iodine after surgery. With the increasing realization that the prognosis for many patients with small differentiated tumors is excellent, there is a growing tendency to be more selective with respect to the extent of surgery and the need for radioactive iodine therapy, as discussed in Selective use of radioactive iodine in the postoperative management of patients with papillary and follicular thyroid carcinoma. J Surg Oncol. 2006 Dec 15;94(8):692-700. Review. and Selective surgery and adjuvant therapy based on risk classifications of well-differentiated thyroid cancer. J Surg Oncol. 2006 Dec 15;94(8):678-82. and Is Adjuvant Therapy Useful in Patients with Papillary Carcinoma Smaller Than 2 cm? Thyroid. 2007 Nov 14; [Epub ahead of print.

Indeed, the benefit of radioactive iodine therapy for "low risk" patients with thyroid cancer is difficult to prove. Low-risk patients were defined according to the American Thyroid Association (ATA) and European Thyroid Association(ETA) criteria as follows: complete tumor resection, multifocal pT (pathological assessment of the primary tumor) 1 less than 1 cm, pT1 greater than 1 cm, pT2, pN0, M0, using American Joint Committee on Cancer/Union for International Cancer Control staging sixth edition (10), corresponding to stage 1 for patients under 45 yr old and stages 1 and 2 for patients over 45 yr old.Impact on Overall Survival of Radioactive Iodine in Low-Risk Differentiated Thyroid Cancer Patients J Clin Endocrinol Metab. 2012 Feb 16.

In some retrospective analyses, only older patients with larger tumors appeared to realize a survival benefit from radioactive iodine therapy Survival in patients with papillary thyroid cancer is not affected by the use of radioactive isotope J Surg Oncol. 2007 Jul 1;96(1):3-7. Increasingly, patients with a very low or undectable thyroglobulin after thyroid cancer surgery and TSH stimulation may, in some centres, elect to defer radioactive iodine, and be followed simply with period assessments and L-thyroxine therapy Postoperative Stimulated Thyroglobulin ≤ 1 ng/ml as a Criterion to Spare Low-Risk Patients with Papillary Thyroid Cancer from Radioiodine Ablation Thyroid. 2012 Aug 17

Nevertheless, it should be realized that there is considerable variability in the behavior of thyroid cancers and even small tumors, referred to as microcarcinomas, may sometimes behave as extensively as larger tumors, as outlined in Aggressiveness and outcome of papillary thyroid carcinoma (PTC) versus microcarcinoma (PMC): A mono-institutional experience. J Surg Oncol. 2007 Jan 16; [Epub ahead of print]. Several studies also suggest that multifocal or multicentric tumors may recur with higher frequency High Recurrent Rate of Multicentric Papillary Thyroid Carcinoma Ann Surg Oncol. 2009 Jun 16. [Epub ahead of print], hence this type of pathology may also influence the decision as to use of radioactive iodine. Alternatively there are several reports describing excellent outcomes for patients with small thyroid cancers treated with surgery only, but no radioactive iodine Excellent Prognosis of Patients with Solitary T1N0M0 Papillary Thyroid Carcinoma Who Underwent Thyroidectomy and Elective Lymph Node Dissection Without Radioiodine Therapy World J Surg. 2009 Dec 30. [Epub ahead of print]. In some centers, there is a trend towards assessment of the thyroglobulin level after surgery as an additional parameter guiding use of radioactive iodine, as a low Tg post ablation is associated with a very low risk of disease recurrence Post-ablative serum thyroglobulin is an independent predictor of recurrence in low-risk differentiated thyroid carcinoma - a 16-yr follow-up study Eur J Endocrinol. 2010 Sep 2. [Epub ahead of print] and Serum Thyroglobulin Levels at the Time of 131I Remnant Ablation just after Thyroidectomy are Useful for Early Prediction of Clinical Recurrence in Low-risk Patients with Differentiated Thyroid Carcinoma. J Clin Endocrinol Metab. 2004 Dec 21; [Epub ahead of print].

For small thyroid cancers confined to the thyroid, there is a gradual trend towards less use of radioactiodine after surgery Decreasing use of radioactive iodine for low risk thyroid cancer in California, 1999-2015 J Clin Endocrinol Metab. 2017 Dec 15. doi: 10.1210/jc.2017-02269. and

Trends in the management of localized papillary thyroid cancer in the United States (2000-2018) Thyroid. 2022 Jan 25. doi: 10.1089/thy.2021.0557


I just had my thyroid surgery. When can I have my radioactive iodine treatment?

In order for the radioactive iodine treatment to be effective and destroy any remaining thyroid tissue, the levels of TSH, secreted by the pituitary gland must rise to elevated levels. TSH, also known as thyroid stimulating hormone, stimulates the thyroid cells to take up the radioactive iodine. For the levels of TSH to rise, the levels of thyroid hormone must have dropped to very low levels. As thyroid hormone has a long half life, it takes many weeks for levels to drop and TSH levels to rise. Accordingly, the earliest time interval that radioactive iodine can be effectively administered is 4 weeks after surgery (complete removal of the thyroid) if the patients has stopped taking all thyroid hormone supplements. To avoid prolonged hypothyroidism, some patients will use Thyrogen. Others will take Cytomel (T3), which must be stopped ~ 14 days prior to the radioactive iodine treatment. Hence, although many patients are understandably anxious to complete their treatment as soon as possible, it is quite common for radioactive iodine to be administered from 4-8 weeks after surgery has been completed. Furthermore, for the vast majority of patients with well-differentiated thyroid cancer, there is no evidence that delaying the radioactive iodine treatment by a few weeks will have any adverse effect on the outcome of treatment. Several small pilot studies have suggested that stopping thyroxine for 3 weeks in adults may be sufficient in 85% of patients Three-week thyroxine withdrawal thyroglobulin stimulation screening test to detect low-risk residual/recurrent well-differentiated thyroid carcinoma. J Endocrinol Invest. 2003 Oct;26(10):1023-31, or weeks in children Children with Differentiated Thyroid Cancer Achieve Adequate Hyperthyrotropinemia within 14 Days of Levothyroxine Withdrawal. J Clin Endocrinol Metab. 2005 Aug 9;, may allow sufficient increases in levels of TSH to achieve satisfactory conditions for radioactive iodine treatment.

I am going to take a vacation in the United States immediately after I finish my treatment. Any special precautions needed?

The United States has recently instituted radioactive monitoring devices at some public buildings and transportation venues, which may be activated by patients recently treated with radioactive iodine. Patients planning on traveling to the US within a month of treatment should take a medical letter from their physician attesting to their diagnosis and treatment. See Radiation monitoring and public safety. A consensus document from the American Thyroid Association that addresses many safety-related questions regarding radioactive iodine treatment for thyroid disease may be accessed through Radiation Safety in the Treatment of Patients with Thyroid Diseases by Radioiodine ((131)I) Practice Recommendations of the American Thyroid Association. Thyroid. 2011 Mar 18. [Epub ahead of print]

One of my friends was prescribed a very low iodine diet for a month prior to her treatment, whereas my physician did not mention this. Is a low iodine diet important prior to radioactive iodine administration?

The theory underlying a low iodine diet prior to treatment is that relative iodine "deficiency" will enhance the uptake and efficacy of the radioactive iodine treatment. In fact, there is no long term data to support this contention, and the short term data argues against the efficacy of a low iodine diet as a factor in the success of treatment. Hence, it does not seem to be a key factor that affects the success of treatment, and physicians cannot point to any data showing that following a low iodine diet influences survival or disease recurrence. Nevertheless, common sense suggests that individuals with less iodine in their system may take up more of the dose, and perhaps the treatment may be slightly more effective. Since radioactive iodine is generally highly effective at destroying any remaining thyroid cells if the proper regimen and dose has been described, patients should no worry excessively if they have been unable to follow a low iodine diet. For individuals who wish to avoid foods rich in iodine content, review our Low Iodine Diet fact sheet. For a scientific assessment of this issue, see Re-evaluation of the impact of a stringent low-iodine diet on ablation rates in radioiodine treatment of thyroid carcinoma. Thyroid. 2001 Aug;11(8):749-55.

I am worried about being isolated in a room by myself. What if I get seriously ill and need help?

For many patients, this may be the first time that one is actually "confined" alone to a room for ~ 48 hours, and the concept that one must stay in a room and not have the freedom to leave is perhaps disturbing and unpleasant. Patients will have a telephone and a television in their rooms to maintain contact with the outside world. It is possible for acquaintances to stand outside the door and talk to patients if desired. Furthermore, patients should be reassured that they will not be neglected if they require medical attention during their hospital stay in isolation.

In some instances, it may be possible to have radioactive iodine administered as outpatient therapy in your own home. To obtain the TGH/UHN guidelines for this outpatient therapy, please review and complete the Outpatient Radioactive Iodine Information and Instructions Forms

What should I wear during my radioactive iodine treatment?

As small amounts of radioactive iodine in body secretions may contaminate your own clothes, underwear or gowns etc, it is advisable to leave your personal clothes in the room closet and us a hospital gown for the duration of your inpatient treatment. Similarly, due to the risk of contamination and the fact that sometimes things go missing from the hospital, leaving all jewelry at home may be a prudent strategy. Hence, if their is any radioactive contamination of the hospital gown, it can simply be sequestered appropriately upon completion of the treatment and discharge. Blankets and sheets will be provided by the hospital.

I have heard there is a drug I can take for my radioactive iodine treatment so I don't have to become hypothyroid. What are the details?

Recombinant TSH, also known as Thyrogen, is approved for diagnostic testing in patients with previously treated thyroid cancer. It is also approved for the therapeutic use of thyroid ablation after surgery in many countries. There is limited experience with Thyrogen in the  treatment of thyroid cancer recurrence. In very rare cases, when the TSH levels cannot be elevated through the normal mechanism of inducing hypothyroidism, recombinant TSH may be used to treat thyroid cancer. There is emerging data that recombinant TSH can be used in the post-operative setting to ablate the thyroid remnant, there is very little data to know how effective this strategy will be. See Radioiodine ablation of thyroid remnants after preparation with recombinant human thyrotropin. Thyroid. 2001 Sep;11(9):865-9 and Radioiodine ablation and therapy in differentiated thyroid cancer under stimulation with recombinant human thyroid-stimulating hormone. J Endocrinol Invest. 2002 Jan;25(1):44-52 and A Retrospective Review of the Effectiveness of Recombinant Human TSH as a Preparation for Radioiodine Thyroid Remnant Ablation. J Nucl Med. 2002 Nov; 43(11): 1482 -1488. Similarly, there is some evidence that ablation of thyroid remnant using recombinant TSH may be associated with less radiotoxicity to other organs Preparation with Recombinant Human Thyroid-Stimulating Hormone for Thyroid Remnant Ablation with 131I Is Associated with Lowered Radiotoxicity J Nucl Med. 2008 Oct 16. [Epub ahead of print]. Furthermore, studies of the 'quality of life' surrounding thyroid remnant ablation suggest that many patients have a higher quality of life isf they avoid hypothyroidism Quality of life changes and clinical outcomes in thyroid cancer patients undergoing radioiodine remnant ablation with recombinant human thyrotropin: a randomized controlled study Clin Endocrinol (Oxf). 2008 Sep 17. [Epub ahead of print]

Furthermore, not all studies using recombinant TSH in conjunction with radioactive iodine to ablate thyroid remnants in patients with thyroid cancer have shown that this treatment is as good as conventional "hypothyroid" ablation in hypothyroid patients. See Ablation of Thyroid Residues with 30 mCi (131)I: A Comparison in Thyroid Cancer Patients Prepared with Recombinant Human TSH or Thyroid Hormone Withdrawal. J Clin Endocrinol Metab. 2002 Sep;87(9):4063-8. Some investigators feel that ongoing thyroxine therapy delivers a daily dose of iodine to the patient, and this reduces the effectiveness of the destruction of remnant thyroid tissue when thyroxine is given with recombinant TSH. See Radioiodine treatment with 30 mCi after recombinant human thyrotropin stimulation in thyroid cancer: effectiveness for postsurgical remnants ablation and possible role of iodine content in L-thyroxine in the outcome of ablation. J Clin Endocrinol Metab. 2003 Sep;88(9):4110-5.

Furthermore, some studies show that withdrawal from thyroid hormone may be more effective for delivery of radioactive iodine into residual tumor tissues, as described in Comparison of iodine uptake in tumour and nontumour tissue under thyroid hormone deprivation and with recombinant human thyrotropin in thyroid cancer patients. Clin Endocrinol (Oxf). 2006 Oct;65(4):519-23. In contrast, the frequency of chromosomal damage in circulating white blood cells (lymphocytes) may be less frequent in patients using recombinant TSH, as opposed to thyroid hormone withdrawal, in conjunction with radioactive iodine therapy Chromosome translocation frequency after radioiodine thyroid remnant ablation: a comparison between rhTSH stimulation and prolonged levothyroxine withdrawal. J Clin Endocrinol Metab. 2009 Jun 9. [Epub ahead of print]. There may also be differences in the radioactive iodine excretion rates, retention in the thyroid remnant, and exposure to non-thyroid tissues, when using recombinant TSH vs. withdrawal, however the clinical significance of these differences is not clear Iodine Biokinetics and Radioiodine Exposure after Recombinant Human Thyrotropin-Assisted Remnant Ablation in Comparison with Thyroid Hormone Withdrawal J Clin Endocrinol Metab. 2010 Apr 14. [Epub ahead of print]. Ongoing analysis of the short term comparative effectiveness of using recombinant TSH vs withdrawal for patients with metastatic thyroid cancer shows very little difference in the relative efficacy or the side effect profile of the two methods for preparing patients with known metastatic disease RADIOIODINE TREATMENT OF METASTATIC THYROID CANCER: RELATIVE EFFICACY AND SIDE EFFECT PROFILE AFTER PREPARATION BY THYROID HORMONE WITHDRAWAL VS. RECOMBINANT HUMAN TSH Thyroid. 2011 Dec 22.

Although extensive long term follow-up data is limited, the available data do not indicate a difference in disease outcome in patients who received Throgen, vs. those who became hypothyroid, for radioactive iodine therapy, as determined after 5 years of follow-up in a small group of patients. Five-Year Survival Is Similar in Thyroid Cancer Patients with Distant Metastases Prepared for Radioactive Iodine Therapy with either Thyroid Hormone Withdrawal or Recombinant Human TSH J Clin Endocrinol Metab. 2011 May 11. [Epub ahead of print]. Short term follow up of thyroid ablation in patients with more extensive thyroid cancer at initial presentation also shows simlar rates of iniitial efficacy (ablation of functioning thyroid cells) in subjects who withdrew from thyroid hormone vs. those who received recombinant TSH High-Risk Patients with Differentiated Thyroid Cancer T4 Primary Tumors Achieve Remnant Ablation Equally Well Using rhTSH or Thyroid Hormone Withdrawal Thyroid. 2013 Sep 16.

Hence the optimal method of administration, and potential efficacy of recombinant TSH, in this setting, requires a discussion between patient and physician and requires ongoing further evaluation.

I needed to have a CT scan before my radioactive treatment-will that affect the effectiveness of the therapy?

Some imaging/rdiology procedures may be carried out in association witht he administration of oral or intravenous contrast material that may contain iodine and theoretically block the subsequent uptake of radioactive iodine meant to destroy your thyroid remnant and any remaining thyroid cancer cells. While some centers recommend waiting 3-6 months after receiving contrast before proceeding with radioactive iodine, one study shows that by 1 month, urinary iodine excretion may be very similar to that measured at 6 months. The exact timing of the treatment should be discussed with your health care team The impact of iodinated contrast agent administered during preoperative CT scan on body iodine pool in patients with differentiated thyroid cancer preparing for radioiodine treatment Thyroid. 2013 Dec 2.

How will I feel when I am off thyroid hormone for several weeks?

The answer to this question is highly patient-specific. Some patients feel minimal fatigue and no major discomfort, despite having very low levels of thyroid hormone. It is not uncommon for some patients, depending on their occupation, to be able to work right up to their time of admission for radioactive iodine treatment. In contrast, other patients may experience considerable fatigue, aches and pains, muscle stiffness, fluid retention, changes in mood, and difficulty with memory or concentration, and find the hypothyroid state extremely unpleasant and debilitating. These latter patients may often comment that they had no prior idea how important thyroid hormone was prior to this experience. In summary, the response of patients to withdrawal of thyroid hormone is variable, but most patients will exhibit some degree of discomfort when withdrawn from their normal levels of thyroid hormone. See J Clin Endocrinol Metab. 2001 Aug;86(8):3864-70.

When can I go back to work after my radioactive iodine treatment?

The answer depends on the individual response to treatment, your type of work, and how well the associated hypothyroidism is tolerated. For otherwise young healthy patients who are self-employed and not in close intimate contact with co-workers, some patients elect to start work immediately, as they feel relatively well and want to keep busy. In other circumstances, such as patients who look after young children, teachers, or persons who work closely with many individuals, or individuals who are quite unwell as a result of the associated hypothyroidism, several additional days or weeks off following discharge from hospital may be prudent. Some patients, particularly older individuals or patients with co-existing medical problems, may feel extremely tired and unwell as a result of sub-optimal thyroid hormone levels and may not return to work for 3-4 weeks.

What kind of contact can I have with other people after my radioactive iodine treatment?

All patients are checked prior to discharge from hospital to ensure that the levels of radiation in their neck and body have returned to levels that are considered safe by regulatory authorities. Using common sense, it seems reasonable to avoid intimate contact for a few days (kissing, hugging, sexual activity, sharing food etc) as there may still be small trace amounts of radioactive iodine in saliva or other body fluids and secretions. Accordingly, while there is no reason for patients to remain secluded in separate rooms after discharge from hospital, intimacy and immediate proximity to others should gradually be reintroduced after several days. There are no scientific studies that have examined whether it is safe to kiss someone at 48 versus 24 hours etc, so we empirically suggest waiting an additional 2-3 days after discharge before resuming normal intimate human activities. To review some data that addresses the disappearance of radioactive iodine from our bodies, see Effective half-life of 131I in thyroid cancer patients. Health Phys. 2001 Sep;81(3):325-9.

How can I be sure that I won't expose my family to radioactive iodine after I come home?

Proper monitoring of patients prior to discharge generally ensures that patients are not overtly radioactive at a reasonable distance, for example, several feet away. For an overview of the type of technical studies that have been done to assess this issue, see Hospital discharge of patients with thyroid carcinoma treated with (131)i. J Nucl Med. 2002 Jan;43(1):61-5. It seems reasonable to avoid sharing food and utensils for several days after the radioactive treatment. Similarly intimate contact involving exchange of body fluids may be avoided so as to minimize inadvertent exposure to small amounts of radioactive iodine that may be still present. On the other hand, it is not necessary to maintain strict isolation procedures once one returns home, as simply avoiding close immediate physical contact for a few days seems reasonable.

Will my hair fall out after the radioactive iodine treatment?

Radioactive iodine does not produce hair loss. Nevertheless, hair loss can be associated with changing levels of thyroid hormone, and may be experienced by some patients with hypo or hyperthyroidism. As radioactive iodine treatments involve withdrawal of thyroid hormone and the development of hypothyroidism, some patients may notice a modest degree of hair loss. When hair loss occurs as a result of changing levels of thyroid hormone, it almost always grows back. Nevertheless, as hair grows slowly, it can take months to a year or so before reestablishment of normal hair thickness occurs.

How safe is the radioactive iodine treatment for thyroid cancer?

Surprisingly to some, the long term risks of developing second cancers after radioactive iodine are extremely small. Radioactive iodine treatment of thyroid cancer is not a "new therapy" and has been used for over 50 years. For a historical overview, see The beginnings of radioiodine therapy of metastatic thyroid carcinoma: a memoir of Samuel M. Seidlin, M. D. (1895-1955) and his celebrated patient. Cancer Biother Radiopharm. 1999 Apr;14(2):71-9. However some studies do show a small but statistical increase in additional cancers in patients with thyroid cancer, even in subjects who did not receive radioactive iodine. The Risk of Second Primary Malignancies Up to Three Decades after the Treatment of Differentiated Thyroid Cancer J Clin Endocrinol Metab. 2007 Nov 20; [Epub ahead of print

The low risk of secondary cancers after radioactive iodine may be related to the minimal genetic damage that the radioactive causes to normal cells, compared to other treatments used in cancer therapy such as chemotherapy or radiotherapy. For an example of how this has been studied, see Nucl Med Commun 1999 Oct;20(10):911-7 Estimation of risk based on biological dosimetry for patients treated with radioiodine. Although inadvertent exposure to radioactive iodine, such as in a nuclear accident, can increase the risk of thyroid cancer in exposed subjects, the medical use of radioactive iodine has not been associated with the same risks, and patients who have had their thyroids removed surgically are clearly not at risk for development of a second thyroid cancer. See Thyroid cancer following exposure to radioactive iodine. Rev Endocr Metab Disord. 2000 Apr;1(3):197-203. Review Breast cancers (and thyroid cancers) are both very common in women however the risk of radioactive iodine treatment seems to have a negligible effect on the subseqent risk of developing breast cancer Risk of Breast Cancer in Patients With Thyroid Cancer Receiving or Not Receiving I-131 Treatment: A Nationwide Population-based Cohort Study J Nucl Med. 2015 Dec 30. pii: jnumed.115.164830 and Risk of primary breast cancer in patients with differentiated thyroid cancer undergoing radioactive iodine therapy: a systematic review and meta-analysis Eur J Nucl Med Mol Imaging. 2021 Nov 25. doi: 10.1007/s00259-021-05625-4

Although conventional wisdom suggests that the risk of developing a second cancer after conventional radioactive iodine therapy is low, and that the risk of leukemia increases only after very high dose radioactive iodine administration, a single study from Israel has documented the development of CML in thyroid cancer patients treated with much lower doses of radioactive iodine, but these findings have not yet been independently confirmed in other populations. See Chronic myeloid leukemia following 131I treatment for thyroid carcinoma: a report of two cases and review of the literature. Clin Endocrinol (Oxf). 1995 Nov;43(5):651-4.

Nevertheless, there is also data which suggests that there is an increased risk of developing a second cancer in thyroid cancer patients, and the risk increases with the cumulative exposure to radioactive iodine, as outlined in Second primary malignancies in thyroid cancer patients. Br J Cancer. 2003 Nov 3;89(9):1638-44. and Second primary malignancy risk after radioactive iodine treatment for thyroid cancer: a systematic review and meta-analysis hyroid. 2009 May;19(5):451-7. Similarly, a retrospective analysis of outcomes for children and adolescents treated with radioactive iodine for thyroid cancer revealed a substantial proportion of patients developed a secondary malignancy from 30-50 years after diagnosis, with 15 of 22 deaths (68%) resulted from nonthyroid malignancy. As there is no control group for this study, interpretation of this observation is challenging. See Long-Term Outcome in 215 Children and Adolescents with Papillary Thyroid Cancer Treated During 1940 Through 2008 World J Surg. 2010 Jan 20. [Epub ahead of print].Similarly, the risk of second cancers often decades following radioactive iodine administration for thyroid cancer treatment in children is low but real Association Between Radioactive Iodine Treatment for Pediatric and Young Adulthood Differentiated Thyroid Cancer and Risk of Second Primary Malignancies J Clin Oncol. 2022 Jan 19:JCO2101841. doi: 10.1200/JCO.21.01841

A single case of salivary gland cancer has been reported in a patient that received large amounts of radioactive iodine Salivary gland malignancy and radioiodine therapy for thyroid cancer. Thyroid. 2010 Jun;20(6):647-51.

Does an abnormal body scan after radioactive iodine always herald worrisome news?

No. Although the most likely explanation for abnormal scans may involve the possibility that the thyroid cancer has been detected outside the neck, there are many benign reasons for having a small amount of abnormal iodine uptake outside the thyroid bed, including "false positive scan results" . It is not uncommon for very small amounts of iodine uptake to be detected in the liver, and if constipation is an issue, iodine may be detected in the bowel. False positive results have also been observed in patients with skin wounds or scabs, as documented in False-Positive Findings on (131)I Whole-Body Scans Because of Posttraumatic Superficial Scabs. J Nucl Med. 2002 Feb;43(2):207-209. For an overview, see  False positive 131I whole body scans in thyroid cancer. Br J Radiol. 2000 Jun;73(870):627-35.

Why do I have to wait several days before coming back for my total body scan?

The evidence shows that immediately following high dose administration of radioactive iodine, there is considerable imaging of radioactive iodine in multiple areas of our bodies, as it initially circulates via blood flow in all our organs and blood vessels. The radioactive iodine is only taken up and retained in tissues that express a specific "iodine transporter" protein, such as thyroid cells. After several days, the "background levels" of radioactivity decline considerably, and areas of the body that have retained radioactive iodine in an abnormal manner are more readily detected.

My iodine scan is negative, but my physician wants to re-treatment me with radioactive iodine. Why?

In some cases, there may be a discordance between the results of the scan, which may not show much and either other radiological imaging studies, the clinical exam, pathology report or results of the thyroglobulin blood test. Hence it is not unusual for patients to be treated with radioactive iodine, even if an initial diagnostic total body scan shows no significant evidence for abnormal iodine uptake.

My friend had thyroid cancer, and was treated with a single outpatient dose of radioactive iodine. Why do I have to be admitted to hospital?

Although some centers use low dose radioactive iodine treatment protocols for ablation of the thyroid remnant, some evidence demonstrates that a larger dose (more than 30 mCi in Ontario), which generally requires transient hospitalization for safety reasons in Ontario, is more effective in completely ablating any remnant thyroid tissue. For example, see Ablation of the thyroid remnant and 131I dose in differentiated thyroid cancer Clin Endocrinol (Oxf) 2000 Jun;52(6):765-773. However, low dose radioactive iodine can be effective in many individuals and is associated with fewer side effects compared to higher dose treatments as outlined in one comparative study Low vs. high radioiodine activity to ablate the thyroid after thyroidectomy for cancer: a randomized study. PLoS ONE. 2008 Apr 2;3(4):e1885. A non-randomized trial experience also reported that 94% of patients with stage T3 and/or N1 PTC experienced satisfactory remnant ablation with 30 mCi of 131-I  Thyroid Ablation with 1.1 GBq (30 mCi) Iodine-131 in Patients with Papillary Thyroid Carcinoma at Intermediate Risk for Recurrence Thyroid. 2013 Nov 27.

For an overview of expected ablation rates using lower doses of radioactive iodine in an outpatient setting, see Radioiodine dose for remnant ablation in differentiated thyroid carcinoma: a randomized clinical trial in 509 patients. J Clin Endocrinol Metab. 2004 Apr;89(4):1666-73. To review the safety of outpatient radioactive iodine treatment for thyroid cancer, with respect to exposure to other individuals and family pets, see Radiation exposure from outpatient radioactive iodine (131I) therapy for thyroid carcinoma. JAMA. 2000 May 3;283(17):2272-4 and Outpatient therapeutic 131I for thyroid cancer. J Nucl Med Technol. 2005 Mar;33(1):28-30

Outpatient radioactive iodine treatment for thyroid cancer is available to patients in some Toronto centres if they meet specific treatment requirements.

For comparative purposes in comparing units of radioactivity, 1100 MBq = ~ 30 MCi. Outpatient radioactive treatment of thyroid cancer has also been explored in several Canadian Centers, as outlined in Outpatient treatment of thyroid cancer using high doses of iodine 131. Can Assoc Radiol J. 1999  50(5):331-6. Although the establishment of the precise dose such as 30 MCi for utpatient treatment and ablation is based on many presumptions, The AEC/NRC 30 mCi Rule: Regulatory Origins and Clinical Consequences for 131I Remnant Ablative Doses Thyroid. 2014 Aug 20. it seems to be reasonably effective.

For patients who decide to have outpatient treatment, the issue of local contamination of their room and environment still needs to be seriously considered, as outlined in Iodine-131 contamination from thyroid cancer patients. J Nucl Med. 1992 Dec;33(12):2110-5. Nevertheless, although long term follow-up data is lacking, some reports have demonstrated the utility of lower dose outpatient radioactive iodine for the ablation of the remaining thyroid lobe after hemi-thyroidectomy and surgical removal of only one lobe of the thyroid. See Radioiodine lobar ablation as an alternative to completion thyroidectomy in patients with differentiated thyroid cancer. Nucl Med Commun. 2003 Feb;24(2):203-8. For an assessment of the safety of radioactive iodine at home in one study, see

Outpatient radioiodine therapy for thyroid cancer: a safe nuclear medicine procedure Clin Nucl Med. 2011 Jun;36(6):440-5

I have heard that after a diagnostic outpatient scan, the effectiveness of subsequent inpatient treatment with larger doses of radioactive iodine may be diminished. Is this a problem?

Some experts have questioned whether the thyroid gland may be "stunned" after a diagnostic dose of 131-I, leading to diminished uptake of a subsequent therapeutic dose shortly thereafter. Although there may be occasional patients in which a small outpatient 5 mCi dose may have a small effect on subsequent thyroid uptake of radioactive iodine, this does not seem to be a common problem, as reviewed in Absence of thyroid stunning after diagnostic whole-body scanning with 185 MBq 131I. J Nucl Med. 2000 41(7):1198-202 and in The nonimpact of thyroid stunning: remnant ablation rates in (131)i-scanned and nonscanned individuals. J Clin Endocrinol Metab. 2001 86(8):3507-11. Nevertheless, wherever possible, patients should avoid recent exposure to large amounts of iodine, as may occur in the setting of such tests such as CT scans or other imaging modalities that sometimes involve administration of iodine-containing contrast media. Analysis of the clinical importance of the potential stunning effect in several hundred patients with thyroud cancer who did, or did not receive a small diagnostic dose of radioactive iodine 6 days prior to a subsequent treatment 'ablation' dose, revealed no evidence for differences in remnant ablation or tumor recurrence after 3 years of follow-up No adverse affect in clinical outcome using low pre-ablation diagnostic 131I activity in differentiated thyroid cancer: refuting thyroid stunning effect J Clin Endocrinol Metab. 2014 Apr 24:jc20141405

I want to get pregnant and I have been told to wait for a year after receiving my radioactive iodine treatment for thyroid cancer. Is this correct?

Menstrual cycles can often be abnormal for up to a year following radioactive iodine, as described in Ovarian function after radioiodine therapy in patients with thyroid cancer. Exp Clin Endocrinol Diabetes. 2005 Jun;113(6):331-3. Generally, most patients are advised to wait for 6-12 months after their last therapeutic dose of radioactive iodine before becoming pregnant. Although the risk of congenital malformations or low birth weight infants does not appear to be increased, an increased risk of miscarriage has been observed in the first 6-12 months following radioactive iodine treatment in some but not all studies. For representative studies, see J Nucl Med 1996 Apr;37(4):606-12 Exposure to radioactive iodine-131 for scintigraphy or therapy does not preclude pregnancy in thyroid cancer patients and Outcome of pregnancy after radioactive iodine treatment for well differentiated thyroid carcinomas. J Endocrinol Invest. 1998 Nov;21(10):662-7 and Prognosis for fertility and ovarian function after treatment with radioiodine for thyroid cancer. Postgrad Med J. 2002 Feb;78(916):92-3. 

The experience of most centers has been highly favorable with pregnancy outcomes following thyroid cancer treatment. Although ovulation and menstrual periods may be slightly abnormal in the year immediately following radioactive iodine administration, ovarian failure is extremely rare, and there does not appear to be an increased risk of congenital abnormalities in babies born to this population of women. See Prognosis for fertility and ovarian function after treatment with radioiodine for thyroid cancer. Postgrad Med J. 2002 Feb;78(916):92-3. Similarly, a retrospective analysis of women with differentiated thyroid cancer who have given birth revealed no problems with pregnancy or the health of the children, as described in Pregnancy outcome after diagnosis of differentiated thyroid carcinoma: no deleterious effect after radioactive iodine treatment. Int J Radiat Oncol Biol Phys. 2004 Jul 15;59(4):992-1000.

Patients should also be aware that ovarian function and ovulation may be abnormal within the first year after radioactive iodine administration, particularly in older women. See Temporary ovarian failure in thyroid cancer patients after thyroid remnant ablation with radioactive iodine. J Clin Endocrinol Metab. 1989 Jul;69(1):186-90.

Taken together, review of data from studies examining the effects of radioactive iodine in women of child-bearing age find little influence of radioiodine therapy on pregnancy outcome, menstrual function, or long term fertility. See A systematic review examining the effects of therapeutic radioactive iodine on ovarian function and future pregnancy in female thyroid cancer survivors Clin Endocrinol (Oxf). 2008 Feb 13; [Epub ahead of print]

I am breast feeding and I need a radioactive iodine treatment. What should I do?

The timing of the treatment depends on the type and stage of your thyroid cancer, and there is no correct fixed answer that suits all patients. Additional factors that need to be considered include the importance and benefits of breast feeding, versus the psychological implications of deferring cancer treatment. Since the majority of well differentiated thyroid cancers are slow growing, and often surgery alone may be highly effective initial treatment, it is not possible to state with 100% certainty that all women should stop breast feeding immediately to have radioactive iodine treatment. This decision should be discussed with your physician. As traces of radioactive can linger for days to weeks in breast milk, the advisability and timing of reinstitution of breast feeding after radioactive iodine treatment should also be discussed with a physician.

I want to have a baby after my radioactive iodine treatment. Is this advisable?

Thousands of women have had successful pregnancies after receiving treatment for thyroid cancer with radioactive iodine. Most centers advocate waiting a year after therapeutic radioactive iodine exposure, although the precise interval required to ensure "safety" has not been optimally studied. To review some of the series of reports that examine this issue, see Outcome of pregnancy after radioactive iodine treatment for well differentiated thyroid carcinomas. J Endocrinol Invest. 1998 Nov;21(10):662-7 and Conception after iodine-131 therapy for differentiated thyroid cancer. Thyroid. 1998 Nov;8(11):1009-11 and Pregnancy after high therapeutic doses of iodine-131 in differentiated thyroid cancer: potential risks and recommendations. Eur J Nucl Med. 1993 Mar;20(3):192-4.

What can I do to prevent side effects from the radioactive iodine treatment?

The majority of radioactive iodine is taken up by your thyroid cells, but some uptake may also occur in salivary glands, ovaries and testicles, lactating breast, and the lining of the stomach. Common side effects include stomach upset, and some degree of neck tenderness, and reduced sense of taste for a few weeks if saliva is diminished. See Multifactorial analysis on the short-term side effects occurring within 96 hours after radioiodine-131 therapy for differentiated thyroid carcinoma. Ann Nucl Med. 2004 Jun;18(4):345-9. Drinking reasonable amounts of fluids should help "flush" the radioactive iodine out of your system. Similarly, making sure that you are not constipated should prevent excess concentration of the iodine in one spot of your gastrointestinal tract, which may show up as a "false positive" spot on a subsequent body scan.. Chewing candies or gum during your hospitalization to keep your salivary gland function active may also reduce the amount of radioactivity taken up by the salivary glands. Importantly, there is some data that suggests that the candies or gum should not be used until 24 hrs after administration of the radioactive iodine. See Does lemon candy decrease salivary gland damage after radioiodine therapy for thyroid cancer?J Nucl Med. 2005 Feb;46(2):261-6. and The role of radioactive iodine in salivary gland dysfunction. Ear Nose Throat J. 2000 Jun;79(6):460-8.

Are there any "delayed complications" following radioactive iodine treatment that may occur months to years later?

Salivary gland inflammation or dysfunction has been reported, which may occur months to a year or so later. This may manifest as decreased saliva, altered taste sensation, or rarely, gum or teeth problems. These problems, although mild in most patients, are probably more common than generally appreciated, as outlined in Intermediate and long-term side effects of high-dose radioiodine therapy for thyroid carcinoma. J Nucl Med. 1998 Sep;39(9):1551-4. Some patients will develop acute severe inflammation of the parotid gland, and present with a swollen face on one or both sides, similar to the picture that develops in patients who have the viral infections known as "mumps". For an overview, see The role of radioactive iodine in salivary gland dysfunction. Ear Nose Throat J. 2000 Jun;79(6):460-8. Review and Radioactive iodine and the salivary glands. Thyroid. 2003 Mar;13(3):265-71. and Salivary Gland Function Five Years after a Radioiodine Ablation in Patients with Differentiated Thyroid Cancer: Direct Comparison of Pre and Post-Ablation Scintigraphies and Their Relation to Xerostomia Symptoms. Thyroid. 2012 Nov 15.

Dry eyes due to changes in tear production, or paradoxically increased tearing, due to blockage of the tear ducts that "drain" tears away from the eyes, has also been reported. See Nasolacrimal drainage system obstruction from radioactive iodine therapy for thyroid carcinoma. J Clin Endocrinol Metab. 2002 Dec;87(12):5817-20 and Nasolacrimal Obstruction Secondary to I131 Therapy. Ophthal Plast Reconstr Surg. 2004 Mar;20 (2): 126-129.

A decrease in white blood cell counts, predominantly neutrophils and lymphocytes, may occur and persist for several months after radioactive iodine treatment. Interestingly, in a small retrospective analysis of patients with thyroid cancr and diabetes, individuals taking metformin had significantly less reduction in their WBC counts after radioactive iodine.

Metformin Attenuates 131I-Induced Decrease in Peripheral Blood Cells in Patients with Differentiated ThyroidCancer. Thyroid. 2015 Dec 30.


Is lithium used routinely to enhance the effect of the radioactive iodine treatment?

There is some experimental clinical evidence to suggest that lithium may enhance retention of radioactive iodine in thyroid tissue following administration for the treatment of thyroid cancer. While still experimental, this type of combination therapy may be useful in selected patients. See Lithium as a potential adjuvant to 131I therapy of metastatic, well differentiated thyroid carcinoma. J Clin Endocrinol Metab. 1999 Mar;84(3):912-6. and evidence from small short term studies supports the possibility that use of lithium at the time of radioactive iodine administration may enhance the effectiveness of the radioactive iodine treatment for remnant ablation Lithium as an adjuvant in the postoperative ablation of remnant tissue in low-risk thyroid carcinoma Thyroid. 2012 Oct;22(10):1002-6

My thyroid cancer has spread to my bones. Will the radioactive iodine treatment still work?

Patients with well differentiated thyroid cancer often have an excellent response to radioactive iodine, even when the disease has spread to the skeleton. For an overview of one center's results, see Outcome after radioiodine therapy in 107 patients with differentiated thyroid carcinoma and initial bone metastases: side-effects and influence of age. Eur J Nucl Med. 2001 Feb;28(2):203-8.

My thyroglobulin level is low but still detectable and my doctor wants to treat me again with radioactive iodine. Is this necessary?

Some  patients will have a low thyroglobulin (Tg) level while taking thyroxine, but the level of Tg will increase significantly when thyroxine is withdrawn or a patient is given recombinant TSH. In patients with an abnormal Tg rise, this can indicate the presence of occult thyroid cancer. For the results of one study where such patients were treated with radioactive iodine, see Efficacy of high therapeutic doses of iodine-131 in patients with differentiated thyroid cancer and detectable serum thyroglobulin. Eur J Nucl Med. 2001 Feb;28(2):198-202.

My total body scan showed some degree of radioactive iodine uptake in my liver. Does this mean my cancer has spread to the liver?

Uptake and accumulation of radioactive iodine in the liver after treatment for thyroid cancer is extremely common, and does not imply the presence of residual cancer in the liver or elsewhere. To review studies examining this issue, see Association between Residual Thyroid Carcinoma and Diffuse Hepatic Uptake of 131I following Radioiodine Ablation in Postoperative Total Thyroidectomy Patients and Hepatic visualization on iodine-131 whole-body thyroid cancer scans. J Nucl Med. 1987 Sep;28(9):1408-11.

Should men bank sperm prior to radioactive iodine treatment for thyroid cancer?

There is very little good data that addresses this issue, but some studies have reported a transient decrease in sperm counts in treated subjects, and rare cases of testicular damage in younger males have been reported. See Testicular damage after radioactive iodine (I-131) therapy for thyroid cancer. Clin Endocrinol (Oxf). 1983 May;18(5):465-72 and Gonadal damage due to radioactive iodine (I131) treatment for thyroid carcinoma. Postgrad Med J. 1985 Apr;61(714):361-2. Furthermore, transient decreases in testicular function and sperm counts have also been described in subjects with hyperthyroidism treated with radioactive iodine. Testicular function after (131)I therapy for hyperthyroidism. Clin Endocrinol (Oxf). 2006 Oct;65(4):446-52. Hence, for young men considering a family in the future, who may be exposed to multiple radioactive iodine treatments sperm banking prior to treatment is an option to be considered as discussed in Testicular function after radioiodine therapy in patients with thyroid cancer. Thyroid. 2006 Jul;16(7):667-70. Nevertheless, analysis of fertility and pregnancy outcomes in the setting where the father had received radioactive iodine treatment for thyroid cancer revealed no significant impairment in fertility, nor any problems with the children. See Testicular dose and fertility in men following I131 therapy for thyroid cancer. Clin Endocrinol (Oxf). 2002 Jun;56(6):755-8.

Although the risk of infertility after radioactive iodine is likely extremely small and not well quantified, some patients who need to be treated with high doses of radioactive iodine may elect to undergo sperm banking prior to treatment Testicular function after radioiodine therapy in patients with thyroid cancer. Thyroid. 2006 Jul;16(7):667-70.

How long does it take the radioactive iodine to clear from my body?

There is some degree of individual variation in how patients will eliminate the radioactive iodine in the urine and stool, but in most patients, the majority of administered radioactive iodine has been excreted after 48 h. A recent study estimates the t1/2 of administered radioactive iodine to be about 14 hours, meaning that by 72 h after the first dose, most subjects will have cleared more than 95% of the initial dose. See Effective half-life of 131I in thyroid cancer patients. Health Phys. 2001 Sep;81(3):325-9.

I have kidney failure and I am on dialysis. Can I still be treated with radioactive iodine?

The answer is yes, however special attention should be given to the dose of radioactive iodine administered, and the timing and frequency of dialysis after radioactive iodine administration. See Iodine-131 treatment of thyroid papillary carcinoma in patients undergoing dialysis for chronic renal failure: a dosimetric method. Thyroid. 2001 Nov;11(11):1031-4. and Radioiodine therapy for thyroid cancer and hyperthyroidism in patients with end-stage renal disease on hemodialysis. Thyroid. 2005 Dec;15(12):1321-31 and Follicular carcinoma of the thyroid with aggressive metastatic behavior in a pregnant woman: Report of a case and review of the literature. Hormones (Athens). 2006 Oct;5(4):295-302.

I have only had half my thyroid removed and the pathology report showed cancer. Do I need to have a second operation or can radioactive iodine be used in my situation?

The answer to this question is a matter of some debate. If the pathology report shows a large tumor, with aggressive features such as lymph node involvement, most physicians would strongly recommend completion thyroidectomy and removal of the remaining thyroid lobe. On the other hand, if the pathology shows a small well differentiated thyroid cancer, there is some evidence that low dose outpatient radioactive iodine can be used to ablate the remaining thyroid lobe, as outlined in Radioactive iodine lobe ablation as an alternative to completion thyroidectomy for follicular carcinoma of the thyroid. Thyroid. 2002 Nov;12(11):989-96.

My thyroglobulin blood test was measured at the time I received my radioactive iodine-does this have any prognostic significance?

The thyroglobulin level is a indirect assessment of how many thyroid cells remain, since it is only thyroid cells that can produce thyroglobulin. Several studies suggest that the lower the level of thyroglobulin even prior to treatment with radioactive iodine, the lower the risk of recurrence. For example patients with a thyroglobulin of less than 2 ug/ml after thyroid surgery have a very low risk of disease recurrence, as described in Serum Thyroglobulin Levels at the Time of 131I Remnant Ablation just after Thyroidectomy are Useful for Early Prediction of Clinical Recurrence in Low-risk Patients with Differentiated Thyroid Carcinoma. J Clin Endocrinol Metab. 2004 Dec 21; [Epub ahead of print].

How is the dose of radioactive iodine selected for thyroid remnant ablation?

The does can vary widely, depending on local institutional preference. Generally, the larger the does given the more successful the extent of remnant ablation. Nevertheless, there are very few randomized studies comparing different doses of radioactive iodine for remnant ablation after surgery. For a recent study comparing 50 mCi vs 100 mCi, see A comparison of 1850 MBq (50 mCi) and 3700 MBq (100 mCi) 131-iodine administered doses for recombinant TSH-stimulated postoperative thyroid remnant ablation in differentiated thyroid cancer. J Clin Endocrinol Metab. 2007 Jul 3; [Epub ahead of print]. Nevertheless, there is emerging evidence that lower doses of radioactive iodine may be as effective as conventional hugher doses for the treatment of thyroid cancer. Patients (421, age range, 16 to 80 years) enrolled in the study who had tumor stage T1 to T3, with possible spread to nearby lymph nodes but without metastasis. End points evaluated were the rate of success of ablation at 6 to 9 months, adverse events, quality of life, and length of hospital stay. There was no apparent difference in the success of treatment (ablation of remnant thyroid) in patients who received low vs. high dose radioactive iodine, although long term follow up data is not available. Side effects and hospitalization rates were higher in patients treated with high dose 131-I. Ablation with low-dose radioiodine and thyrotropin alfa in thyroid cancer N Engl J Med. 2012 May 3;366(18):1674-85. Given the lack of long term data (randomized controlled clinical trials comparing low dose vs. high dose radioactive iodine with 10 year follow-up in large groups of patients are not available), treatment decisions need to be made with the best available data considering individual circumstances. Some retrospective analyses report higher rates of recurrence or suboptimal outcomes in higher risk patients (age more than 45 with evidence for aggressive initial disease presentation  Long-term survival in differentiated thyroid cancer is worse after low-activity initial post-surgical (131)i therapy in both high- and low-risk patients J Clin Endocrinol Metab. 2014 Dec;99(12):4487-96