What kind of diet should one follow or what natural supplements can one take to help the thyroid function normally? This is a very common question, and at present, there is little scientific evidence, but much opinion based on non-rigorous poorly designed studies, that surrounds this issue. It is important to remember that much of the evidence linking micronutrients and vitamins to thyroid dysfunction comes from studies of malnourished animals or humans with nutrient deficiencies. Nevertheless, a common question in North America, where most patients do not experience nutrient deficiency, is whether adding exogenous "natural" supplements to the diet will be beneficial for thyroid function. For a comprehensive overview of scientific data available for a particular herb or supplement, see www.herbmed.org.

Peruse a general overview of Vitamin supplementation published in the Wall Street Journal in March 2006

What does the data actually show?

Do normal subjects exhibit a correlation between levels of thyroid hormones and levels of specific micronutrients such as selenium, zinc, retinol, and alpha-tocopherol? A study of Italian subjects of varying ages 20-65 yr) and 44 elderly persons (age range, 65-89) demonstrated no correlation between micronutrients and thyroid hormones in the 20-65 year old group, but in the very old subjects, plasma retinol was negatively associated with Free T4 and TSH serum levels whereas serum zinc was positively associated with serum Free T3 and FT3/FT4 ratio. However, this type of data is correlative only, and does not show cause and effect. See Blood micronutrient and thyroid hormone concentrations in the oldest-old. J Clin Endocrinol Metab. 2000 Jun;85(6):2260-5.

It now seems quite clear that patients with hypothyroidism often have an increase in levels of homocysteine, and this may correlate, in population-based studies, with an increased risk for hypothyroidism. See Plasma total homocysteine levels during short-term iatrogenic hypothyroidism. J Clin Endocrinol Metab. 2000 Mar;85(3):1049-53. and the Section on Heart Disease. Intake of B vitamins through diet, supplementation and fortified foods will increase levels of homocysteine, but there is no evidence that such supplements will modify thyroid function in individuals with thyroid disease.

What is the effect of vitamin E supplementation on thyroid function in 88 healthy subjects aged >65 y participating in a double-blind, placebo-controlled trial? The answer is that there does not appear to be any significant effect in subjects taking 60, 200, or 800 IU of vitamin E for 4 months. See Assessment of the safety of supplementation with different amounts of vitamin E in healthy older adults. Am J Clin Nutr. 1998 Aug;68(2):311-8

Although selenium is clearly important for optimal synthesis of thyroid hormones, and the thyroid gland normally stores both iodine and selenium, there is little scientific evidence from well-designed studies to suggest that selenium supplementation has sustained beneficial effects on thyroid function. Several studies, often non-randomized, have shown modest changes in levels of antibodies, but no clear improvement in thyroid hormone levels as shown in Effects of a six Month treatment with selenomethionine in patients with autoimmune thyroiditis. Eur J Endocrinol. 2003 Apr;148(4):389-393 and Selenium Supplementation in Patients with Autoimmune Thyroiditis Decreases Thyroid Peroxidase Antibodies Concentrations. J Clin Endocrinol Metab. 2002 Apr 1;87(4):1687-1691. and The effect of selenium on thyroid status in a population with marginal selenium and iodine status. Br J Nutr. 2005 Dec;94(6):962-8. Moreover a randomized double-blind, placebo-controlled trial of selenium supplementation in 501 elderly UK volunteers showed no effect of 6 months of selenium supplementation on thyroid hormone status. See Randomized controlled trial of the effect of selenium supplementation on thyroid function in the elderly in the United Kingdom Am J Clin Nutr. 2008 Feb;87(2):370-8. Similarly, a 12 month trial of selenium supplementation in patients with autoimmune thyroiditis demonstrated no clinically significant effect on thyroid hormone status INFLUENCE OF PHYSIOLOGICAL DIETARY SELENIUM SUPPLEMENTATION ON THE NATURAL COURSE OF AUTOIMMUNE THYROIDITIS Clin Endocrinol (Oxf). 2009 Dec 18. [Epub ahead of print]

Furthermore, selenium supplementation can sometimes be toxic, as described in Adverse health effects of selenium in humans. Rev Environ Health. 2001 Jul-Sep; 16(4):233-51.

Similarly, Dietary selenium intake modulates thyroid hormone and energy metabolism in men. J Nutr. 2003 Nov;133(11):3443-8.

For an overview of selenium, see The importance of selenium to human health. Lancet. 2000 Jul 15;356(9225):233-41. 

Non-Prescription over the counter thyroid supplements

Many thyroid supplements are sold over the counter contain surprising amount of thyroid hormone, either T4, T3 or both. Patients need to be aware that these supplements, often sold as "energy supplements" may contain enough thyroid hormone to occasionally cause thyroid dysfunction (usually hyperthyroidism) in susceptible individuals THYROXINE AND TRIIODOTHYRONINE CONTENT IN COMMERCIALLY AVAILABLE THYROID HEALTH SUPPLEMENTSThyroid. 2013 Jun 13.

Oxidative stress and Coenzyme Q

A large number of metabolic intermediaries may be measured in different thyroid states, in both the blood and in the thyroid. These types of studies often show a decreased level of "anti-oxidant metabolites", leading to the suggestion that patients with thyroid disease should take supplements containing molecules with anti-oxidant properties, such as vitamin C, vitamin E, and coenzyme Q10. For example, see Oxidative stress and anti-oxidant metabolites in patients with hyperthyroidism: effect of treatment. Horm Metab Res. 1999 Nov;31(11):620-4. It seems quite likely that too much or insufficient levels of thyroid hormone are clearly associated with changes in the levels of various vitamins and coenzymes that function as anti-oxidants. Nevertheless, at present there is little scientific evidence from well-designed randomized studies of patients with thyroid disease to indicate that these supplements will exert significant beneficial effects on the resolution of thyroid dysfunction in human subjects. Indeed, a non-randomized study of an antioxidant mixture alone, or in combination with antithyroid drugs, showed that patients treated with an anti-oxidant mixture alone did not exhibit a return to normal in levels of thyroid hormones. See Antioxidants in the treatment of Graves disease. IUBMB Life. 2001 Feb;51(2):105-9.

How about soy? While the risks and benefits of soy supplementation for various medical conditions continue to be debated, patients taking large amounts of soy may experience reduced absorption of thyroid hormone and an increased requirement for L-thyroxine, particularly in the setting of iodine deficiency. See Use of soy protein supplement and resultant need for increased dose of levothyroxine. Endocr Pract. 2001 May-Jun;7(3):193-4. and Goitrogenic and estrogenic activity of soy isoflavones. Environ Health Perspect. 2002 Jun;110 Suppl 3:349-53.  In contrast, simply supplementing the diet with exogenous soy isoflavones does not impair thyroid function over several months in postmenopausal women, as illustrated in Isoflavone supplements do not affect thyroid function in iodine-replete postmenopausal women. J Med Food. 2003 Winter;6(4):309-16. Furthermore a review of the literature provides only weak evidence in support of a role for soy in the perturbation of thyroid function in human subjects Effects of soy protein and soybean isoflavones on thyroid function in healthy adults and hypothyroid patients: a review of the relevant literature. Thyroid. 2006 Mar; 16(3): 249-58. Sathyapalan and colleagues studied the consaequences of soy phytoestrogen administration (30 g soy daily) for 8 weeks in patients with subclinical hypothyroidism. Six female patients developed overt hypothyroidism during the short trial, demonstrating the risk of soy supplementation in "at risk" individuals. Hence the benefits and risks of soy supplementation need to be carefully considered in the individual patient The effect of soy phytoestrogen supplementation on thyroid status and cardiovascular risk markers in patients with subclinical hypothyroidism: a randomized, double-blind, crossover study J Clin Endocrinol Metab. 2011 May;96(5):1442-9.

Similarly, patients ingesting increased amounts of fiber in their diets may also require unusually high does of L-thyroxine, as described in Evidence for a clinically important adverse effect of fiber-enriched diet on the bioavailability of levothyroxine in adult hypothyroid patients. J Clin Endocrinol Metab. 1996 Feb;81(2):857-9. Nevertheless, the effects of soy ingestion appear modest in most individuals. See Effect of soy protein on endogenous hormones in postmenopausal women. Am J Clin Nutr. 2002 Jan;75(1):145-53.

What effect does carnitine supplementation have on thyroid function? The data appears to show that L-carnitine can function as an antagonist of thyroid hormone, likely by blocking the entry of thyroid hormone into cells. Indeed, these properties of L-carnitine may be beneficial in hyperthyroid subjects. As hyperthyroidism may be associated with modest carnitine deficiency, the effects of carnitine supplementation were examined in a small randomized trial of women on L-thyroxine supplementation. Modestly beneficial effects of carnitine were seen in this study, as described in Usefulness of l-carnitine, a naturally occurring peripheral antagonist of thyroid hormone action, in iatrogenic hyperthyroidism: a randomized, double-blind, placebo-controlled clinical trial. J Clin Endocrinol Metab. 2001 Aug;86(8):3579-94.

How about Vitamin D? Although not directly involved in synthesis or secretion of thyroid hormones, vitamin D deficiency can produce increases in the levels of parathyroid hormone and increase the risk of osteoporosis. Vitamin D status and redefining serum parathyroid hormone reference range in the elderly. J Clin Endocrinol Metab. 2001 Jul;86(7):3086-90. For an overview of mild vitamin D deficiency, see Vitamin D deficiency and secondary hyperparathyroidism in the elderly: consequences for bone loss and fractures and therapeutic implications. Endocr Rev. 2001 Aug;22(4):477-501.

How about Tiratricol? Patients taking this supplement should be very careful for a variety of reasons, including the possible development of hyperthyroidism, as shown in A Report of Hyperthyroidism Induced By an Over-the-Counter Fat Loss Supplement (Tiratricol). Int J Sport Nutr Exerc Metab. 2003 Mar;13(1):112-6.