Iodine is a micronutrient required for normal thyroid function. Recommended Daily Allowances (RDA) for iodine intake are 150 mcg in adults, 220-250 mcg in pregnant women, and 250-290 mcg in breastfeeding women in the U.S. (1,2). The U.S. diet generally contains enough iodine to meet these needs, with common sources being iodized salt, dairy products, breads, and seafood. During pregnancy and lactation, women require higher amounts of iodine for the developing fetus and infant. The American Thyroid Association recommends that women take a multivitamin containing 150 mcg iodine daily in the form of potassium iodide (KI) (3) during preconception, pregnancy, and lactation to meet these needs (4).
Ingestion of greater than 1,100 mcg of iodine per day (Tolerable Upper Limits for iodine) (1) is not recommended and may cause thyroid dysfunction. During pregnancy and lactation, the recommendations for the upper limit vary and range from 500-1,100 mcg of iodine daily (2). In particular, infants, the elderly, pregnant and lactating women, and individuals with preexisting thyroid disease are susceptible to adverse effects of excess iodine intake and exposure (5). The public is advised that many iodine, potassium iodide, and kelp supplements contain iodine in amounts that are up to several thousand times higher than the daily Tolerable Upper Limits for iodine. The American Thyroid Association (ATA) advises against the ingestion of iodine and kelp supplements containing in excess of 500 mcg iodine daily for children and adults and during pregnancy and lactation. Chronic iodine intake in amounts greater than the Tolerable Upper Limits should be closely monitored by a physician. There is only equivocal data supporting the benefit of iodine at higher doses than these, including a possible benefit for patients with fibrocystic breast disease (6). There is no known thyroid benefit of routine daily iodine doses in excess of the U.S. RDA.
There are a limited number of medical conditions in which the short-term use of high amounts of iodine is indicated. Exceptions for the recommendations to not exceed the Tolerable Upper Limits include closely-monitored patients prescribed Lugol’s solution or SSKI (saturated solution of potassium iodide) in their treatment of severe hyperthyroidism, such as thyroid storm and prior to surgery in patients with Graves’ disease, and individuals in the vicinity of a nuclear power plant who are recommended to take KI in the event of a nuclear accident. SSKI is not indicated nor recommended in individuals with thyroid nodules. Finally, patients receiving the large amounts of iodine in iodinated contrast dyes, as required for radiologic studies, should be monitored for iodine-induced thyroid dysfunction if risk factors are present.
Key points:
– Adequate iodine intake is required for normal thyroid function
– The recommended iodine intake in non-pregnant adults is 150 mcg daily
– Higher iodine intakes are recommended in pregnancy and lactation
– Given a Tolerable Upper Limit of 1100 mcg iodine daily, ingestion of an iodine or kelp supplement containing in excess of 500 mcg iodine daily should not be done
– Certain exceptions to these recommendations include those for specific medical conditions; such individuals should be closely monitored for thyroid dysfunction
References:
1. Food and Nutrition Board, Institute of Medicine. Dietary reference intakes. Washington, D.C.: National Academy Press; 2006.
2. De Groot L, Abalovich M, Alexander EK, Amino N, Barbour L, Cobin RH, Eastman CJ, Lazarus JH, Luton D, Mandel SJ, Mestman J, Rovet J, Sullivan S. Management of thyroid dysfunction during pregnancy and postpartum: an Endocrine Society clinical practice guideline. J Clin Endocrinol Metab 2012;97:2543-65.
3. Leung AM, Pearce EN, Braverman LE. Iodine content of prenatal multivitamins in the united states. N Engl J Med 2009;360:939-40.
4. Stagnaro-Green, A, Abalovich, M, Alexander, E, Azizi, F, Mestman, J, Negro, R, Nixon, A, Pearce, EN, Soldin, OP, Sullivan, S, Wiersinga, W, American Thyroid Association Taskforce on Thyroid Disease During Pregnancy and Postpartum. Guidelines of the American Thyroid Association for the diagnosis and management of thyroid disease during pregnancy and postpartum. Thyroid 2011;21:1081-1125.
5. Leung AM and Braverman LE. Iodine-induced thyroid dysfunction. Curr Opin Endocrinol Diabetes Obes 2012;19:414-419.
6. Office of Dietary Supplements, National Institutes of Health. Dietary Supplement Fact Sheet: Iodine. Available at: http://ods.od.nih.gov/factsheets/Iodine-HealthProfessional (accessed April 23, 2013).
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