Click here for Frequently Asked Questions on Radioactive Iodine.

The principle advantage of Radioactive Iodine treatment for hyperthyroidism is that it tends to have a much higher success rate than medications. Depending on the dose of radioactive iodine chosen, and the disease under treatment (Grave's vs toxic goitre, vs hot nodule etc), success rate in achieving definitive resolution of the hyperthyroidism may vary from 75-100%. A major expected side effect of radioactive iodine in patients with Graves disease is the development of life long hypothyroidism requiring daily treatment with thyroid hormone. Occasionally, some patients may require more than one radioactive treatment, depending on the type of disease present, the size of the thyroid, and the initial dose administered. Many patients are initially unhappy at the thought of having to take a thyroid hormone pill for the rest of their lives. Nevertheless, as thyroid hormone is safe, inexpensive, and easy to take, and is identical to the thyroid hormone normally made by our own thyroid, this therapy is generally extremely safe and very well tolerated by the vast majority of patients.

As radioactive iodine treatment results in destruction of thyroid tissue, there is often a transient period of several days to weeks when the symptoms of hyperthyroidism may actually worsen following radioactive iodine therapy. This generally happens as a result of thyroid hormone being released into the blood following the radioactive iodine-mediated destruction of thyroid cells which contain thyroid hormone. In some patients, treatment with medications such as beta blockers (propranolol, atenolol, etc) may be useful during this period of time. Many patients are able to tolerate the initial few weeks without any problem whatsoever.

Most patients do not experience any difficulty after the radioactive iodine treatment, usually given as a small pill. Occasionally, neck tenderness or a sore throat may become apparent after a few days, if moderate inflammation in the thyroid develops and produces discomfort in the neck or throat area. This is usually transient, and not associated with a fever etc. 

Women who are breast feeding should discontinue breastfeeding for at least a week, and likely longer, following radioactive iodine treatment, as small amounts of radioactive iodine may be found in breast milk even several weeks after the radioactive iodine treatment.

What are the factors associated with a positive outcome after radioactive iodine? The amount of radioactive iodine administered, severity of existing hyperthyroidism, size of the thyroid, and gender are all factors that influence success of treatment, and provide guidance for the suggested dose of radioactive iodine to be administered. In general, the higher the dose prescribed, the more successful the treatment, as defined by resolution of the hyperthyroidism. See Radioiodine treatment of hyperthyroidism-prognostic factors for outcome. J Clin Endocrinol Metab. 2001 Aug;86(8):3611-7. and Evidence for higher success rates and successful treatment earlier in graves' disease with higher radioactive iodine doses Thyroid. 2012 Oct;22(10):991-5

Is pretreatment with antithyroid medications necessary prior to treatment with radioactive iodine? This depends on the individual patient and severity of hyperthyroidism. Patients with mild to moderate hyperthyroidism who tolerate the condition well may not require pretreatment with antithyroid medications such as PTU or methimazole. In fact, there is some evidence that pre-treatment of patients with either Graves' disease or toxic goiter with antithyroid drugs reduces the effectiveness of the subsequent radioactive iodine treatment, as described in Propylthiouracil before 131I therapy of hyperthyroid diseases: effect on cure rate evaluated by a randomized clinical trial. J Clin Endocrinol Metab. 2004 Sep;89(9):4439-44 and Propylthiouracil reduces the effectiveness of radioiodine treatment in hyperthyroid patients with Graves' disease. Thyroid. 2004 Jul;14(7):525-30.

In contrast, patients presenting with severe hyperthyroidism may require institution of medications immediately, particularly if problems such as heart disease are being adversely affected by the hyperthyroidism. Randomized clinical studies show that pre-treatment of patients with antithyroid drugs prior to radioactive iodine results in more severe transient rebound hyperthyroidism, compared to patients who received radioactive iodine without pretreatment. For an overview, see J Clin Endocrinol Metab 1999 Nov;84(11):4012-6 Effect of methimazole pretreatment on serum thyroid hormone levels after radioactive treatment in Graves' hyperthyroidism and The effect of antithyroid drug pretreatment on acute changes in thyroid hormone levels after (131)I ablation for graves' disease. J Clin Endocrinol Metab. 2001 Jul;86(7):3016-21.


Is radioactive iodine expensive? 

The answer to this question depends on the site that orders and administers the radioactive iodine, and may vary widely. At the Toronto General Hospital, the cost for outpatient radioactive iodine for the treatment of hyperthyroidism may range from $250 to $400 CAD, which is not that different from a years supply of medications (PTU or methimazole) used to treat hyperthyroidism.

 Is radioactive iodine safe? How do we know?

Several studies have examined the long term follow-up of patients treated with radioactive iodine for hyperthyroidism. Although some studies show a very small increase in mortality for the first few years after treatment, the long term safety seems excellent, with no excess cancer mortality after decades of follow-up. See Long-term comparative cancer mortality after use of radio-iodine in the treatment of hyperthyroidism, a fully reported multicenter study. J Insur Med. 2001;33(2):138-42.  There does not appear to be any significant excess total cancer mortality in patients after radioactive iodine. For example, see the studies published in JAMA 1998 280:347-355 or in Lancet 1999 353:2111-2115.

However, contrasting data on the rate of cancer incidence have been reported in some studies Increased cancer incidence after radioiodine treatment for hyperthyroidism Cancer. 2007 May 15;109(10):1972-9

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.

For a summary of the possible side effects of radioactive iodine and a discussion of its safety, see Health effects of therapeutic use of 131I in hyperthyroidism. Q J Nucl Med.2000 Dec;44(4):333-9

I received my radioactive iodine last week. When will I feel normal?

The answer depends on many patient-specific variables. For example, patients with extremely large thyroid glands and severe hyperthyroidism may take much longer to achieve resolution of the hyperthyroidism than patients with small thyroid glands and modest hyperthyroidism. Similarly, the larger the dose of radioactive iodine, the more likely that the thyroid gland will be effectively treated. As radioactive iodine can induce considerable inflammation in the thyroid, this leads to leakage of thyroid hormone from the damaged thyroid into the blood. Accordingly, patients symptoms may actually worsen for a few weeks, before they start to improve. In most cases, improvement may be noted by 4-8 weeks after administration of the radioactive iodine, but in more severe cases, it can take longer. Depending on the type of hyperthyroidism present, and other mitigating factors, the time to "feeling normal" can vary considerably, from 1-12 months. These individual issues and expectations should be discussed with your physician.

I am being treated with radioactive iodine for a hot nodule, how effective is this? 

The majority of patients experience a significant reduction in size of the nodule and a decrease in levels of circulating thyroid hormones usually to normal, often within 3-6 months of treatment. In most centers, the development of hypothyroidism in an otherwise normal thyroid gland is rare, and is seen perhaps 10% of the time. See Long-term effect of radioactive iodine on thyroid function and size in patients with solitary autonomously functioning toxic thyroid nodules. Clin Endocrinol (Oxf). 1999 Feb;50(2):197-202.

I would like to receive just the right amount of radioactive iodine to eliminate my hyperthyroidism, but I don't want to become hypothyroid. Can this be done?

This desired outcome is one of the most difficult to achieve in clinical practice. There have been many formulas developed to calculate precisely how much radioactive iodine is required to achieve a perfect outcome, including variables such as the thyroid size, thyroid volume, and radioactive iodine uptake in predictive treatment nomograms. Some studies have reported very appealing results, balancing effective therapy with a reduced risk of hypothyroidism. Striving for Euthyroidism in Radioiodine Therapy of Graves' Disease: A 12-Year Prospective, Randomized, Open-Label Blinded End Point Study. Thyroid. 2011 May 12. [Epub ahead of print].Unfortunately, despite using various parameters as guidance, it remains very difficult to achieve this outcome. In general, the more one tries not to use too much radioactive iodine, the higher the degree of relapse and requirement for a second treatment. Hence, many physicians will state clearly at the outset that hypothyroidism requiring life long thyroid hormone replacement must be an anticipated outcome in this treatment scenario, so as not to mislead patients that a perfect outcome (thyroid normal, no need to take thyroid replacement tablets) will be achieved. Indeed, studies comparing the outcome after fixed dose versus precisely calculated adjusted dose radioactive iodine administration have failed to show any advantage of more complicated radioactive iodine dosing regimens, as shown in A randomized comparison of radioiodine doses in graves' hyperthyroidism. J Clin Endocrinol Metab. 2003 Mar;88(3):978-83. and there is evidence suggesting that an empirical higher dose may lead to higher and faster rates of resolution compared to lower doses Evidence for Higher Success Rates and Successful Treatment Earlier in Graves´ Disease with Higher Radioiodine Doses Thyroid. 2012 Jun 22. [Epub ahead of print]

I received my radioactive iodine and now my TSH is elevated. Will I be on thyroid hormone for the rest of my life?

Some patients will develop transient hypothyroidism after radioactive iodine, whereas in others, usually the majority of patients who receive effective doses, it may be permanent. The group of patients with transient hypothyroidism seem to develop new thyroid antibodies that block thyroid function temporarily. The usual suggestion is for appropriate monitoring, ~ every 4-10 weeks, until thyroid function stabilizes. See Characterization of thyroid-stimulating blocking antibodies that appeared during transient hypothyroidism after radioactive iodine therapy. Thyroid. 2000 Oct;10(10):909-17.

I want to avoid gaining too much weight after the treatment. Is there anything I can do?

Careful regular blood test monitoring after radioactive iodine administration, usually every 4-8 weeks initially, seems prudent to avoid the development of transient hypothyroidism. If there has been significant weight loss in the course of the hyperthyroid illness, it is reasonable to expect some degree of weight gain, with body weight approaching "prehyperthyroid values" in most patients. Institution of thyroid hormone replacement early in the course of treatment with careful attention to optimal hormone replacement may help minimize the potential for excess weight gain. See Is excessive weight gain after ablative treatment of hyperthyroidism due to inadequate thyroid hormone therapy? Thyroid. 2000 Dec;10(12):1107-11.

What will happen to my thyroid gland and neck after the radioactive iodine?

Some patients will experience mild neck discomfort or sore throat for a few days to weeks after radioactive iodine administration. Less commonly more severe neck discomfort may develop that can last for a few weeks. The use of a non-steroidal anti-inflammatory agent can be helpful in these circumstances. The radioactive iodine will induce an inflammatory process in the thyroid that will result in gradual destruction of thyroid tissue and reduction in the size of the gland. The rate of this process depends on the degree of pre-existing thyroid disease, the size of the thyroid gland, and the dose of radioactive iodine administered.

Should children receive radioactive iodine?

The answer to this question is somewhat controversial, however many centers have successfully used radioactive iodine in the pediatric population with good results. See Pediatric Graves' disease: therapeutic options and experience with radioiodine at the University of Mississippi Medical Center. South Med J. 1997 Oct;90(10):1017-22 and The use of radioactive iodine in the management of hyperthyroidism in children. Curr Drug Targets Immune Endocr Metabol Disord. 2001 Nov;1(3):255-64. and A 36-year retrospective analysis of the efficacy and safety of radioactive iodine in treating young Graves' patients. J Clin Endocrinol Metab. 2004 Sep;89(9):4229-33. and An optimal treatment for pediatric graves' disease is radioiodine. J Clin Endocrinol Metab. 2007 Mar;92(3):797-800.

Do individuals who live near nuclear reactors have to take iodine?

Iodine supplements may be administered to patients at risk for exposure to radioactive iodine release from a nuclear reactor accident, as occurred at Chernobyl. See  Potassium iodide for thyroid blockade in a reactor accident: administrative policies that govern its use. Thyroid. 1997 Apr;7(2):193-7.

Should I be treated with antithyroid drugs for several months first, prior to receiving radioactive iodine for Graves' disease?

The answer depends on how you are feeling and the severity of your hyperthyroidism. For example, if you are extremely ill, or have underlying heart disease with abnormal cardiac rhythms and/or symptoms of heart disease such as shortness of breath and chest pain, then pretreatment with antithyroid drugs first may be advisable. If you have mild to moderate hyperthyroidism which you are tolerating well, you may not need pretreatment with antithyroid drugs. In fact, patients pretreated for several months with PTU or methimazole prior to radioactive iodine will often experience a more severe rebound, albeit transient worsening of their hyperthyroidism after radioactive iodine. Discuss your clinical condition and this issue with your physician if you remain uncertain. More recent data suggests that a 48 hr period of discontinuation of antithyroid drugs prior to radioactive iodine may produce less severe rebound hyperthyroidism, and seems to give reasonable treatment results that are comparable to outcomes achieved after withdrawal of thyroid hormone for 1 week. See Two-day Thionamide Withdrawal prior to Radioiodine Uptake Sufficiently Increases Uptake and does not Exacerbate Hyperthyroidism Compared to 7-day Withdrawal in Graves' Disease. Endocr J. 2006 Aug 8; [Epub ahead of print]and The effect of antithyroid drug pretreatment on acute changes in thyroid hormone levels after (131)i ablation for graves' disease. J Clin Endocrinol Metab. 2001 Jul;86(7):3016-21.

I am scheduled for an outpatient radioactive iodine treatment. What is the risk of radiation exposure to my family?

At the dosing limits allowed for prescribing outpatient use of radioactive iodine, there appears to be little risk to family members in the home environment. Patients are encouraged to minimize immediate close contact for a few days, especially of body fluids such as saliva and other excretions. For example, avoid intimate kissing, sharing of food, and hugging and kissing of young children for a few days after the radioactive iodine treatment. A scientific study of this issue is reported in Radiation exposure from outpatient radioactive iodine (131I) therapy for thyroid carcinoma. JAMA. 2000 May 3; 283 (17) :2272-4. Similarly, theoretical analysis of radiation exposure to family members and co-workers shows minimum risk of radiation exposure, however minimizing direct contact for several days after treatment seems prudent. See Potential third-party radiation exposure from outpatients treated with 131I for hyperthyroidism. Med Phys. 2004 Dec;31(12):3194-200.

I am receiving a form of cancer treatment that involves administration of a form of radioactive iodine conjugated to a monoclonal antibody. Will this harm my thyroid?

Although measures are taken to try and protect the thyroid in these situations, hypothyroidism appears to be a common side effect, at least in some early trials. See High-dose myeloablative radioimmunotherapy of mantle cell non-hodgkin lymphoma with the iodine-131--labeled chimeric anti-CD20 antibody C2B8 and autologous stem cell support. Cancer. 2002 Feb 15;94(S4):1363-1372

I am on medication for my hyperthyroidism and I am going to be treated with radioactive iodine-how long do I have to stop my medication before the radioactive iodine treatment?

Although classic advice would suggest stopping the medications (PTU or methimazole) for 5-7 days, some studies suggest discontinuation of medication for only 2 days may still yield satisfactory outcomes-see Two-day Thionamide Withdrawal prior to Radioiodine Uptake Sufficiently Increases Uptake and does not Exacerbate Hyperthyroidism Compared to 7-day Withdrawal in Graves' Disease. Endocr J. 2006 Aug 8; [Epub ahead of print]

Can I take lithium to enhance the effectiveness of radioactive iodine therapy?

Lithium may increase the retention of iodine within the thyroid gland, thereby increasing the efficacy of radioactive iodine. Although lithium is not widely used together with radioactive iodine, emerging data from some studies suggests combining lithium with radioactive iodine might be an effective therapeutic combination Impact of Lithium on Efficacy of Radioactive Iodine Therapy for Graves' Disease: A Cohort Study on Cure Rate, Time to Cure, and Frequency of Increased Serum Thyroxine After Antithyroid Drug Withdrawal. J Clin Endocrinol Metab. 2009 Nov 11. [Epub ahead of print]