Adequate iodine nutrition is necessary for the production of thyroid hormones, and pregnant women and their fetuses are particularly vulnerable to the effects of iodine deficiency. Worldwide, iodine deficiency is considered the leading preventable cause of intellectual impairment. Even mild maternal iodine deficiency has been associated with lower child intelligence and standardized test scores.1, 2
There is good information about iodine nutrition and the thyroid available on the Internet, but unfortunately there is also misinformation. The following are six misperceptions about iodine nutrition and thyroid supplements that patients frequently ask me about:
- The United States is considered iodine-sufficient, so I have no need to worry about my iodine nutrition.
- Sea salt is a good source of dietary iodine.
- It is safe to assume that my prenatal multivitamin contains adequate iodine.
- Measurement of iodine in my urine (or blood, hair or fingernails) can determine whether my intake is adequate.
- If some iodine in the diet is good, more must be better.
- Supplements marketed for thyroid support are a safe source of iodine nutrition.
While the U.S. has been iodine-sufficient overall for decades, pregnant women in the U.S. have been mildly iodine-deficient in recent national surveys.3
Vegans4, individuals with lactose intolerance and others who do not consume dairy products may also be at risk. It is difficult to provide dietary advice to patients regarding iodine intake since iodine content is not labeled on most food packaging in the U.S., and food iodine content can be quite variable. However, dairy foods are currently the major source of iodine in the U.S. diet, and an 8-oz glass of milk contains about 110 mcg iodine.5 Iodized salt and seafood are other good nutritional sources of iodine.
Salt iodization has been a mainstay of iodine deficiency disorder prevention efforts since the 1920s. However, salt iodization has never been mandated in the U.S. Sea salt does not naturally contain a significant amount of iodine. Although iodine is not added to most sea salts marketed in the U.S., some iodized sea salts are available. It is important to check packaging.
Pregnant women need higher iodine intake than nonpregnant adults because of increased thyroid hormone production, transfer of iodine to the fetus and increased renal iodine losses. The recommended dietary allowance (RDA) for iodine is 220 mcg per day in pregnancy, which is higher than the 150 mcg daily recommended for nonpregnant adults.
Importantly, iodine requirements remain high for women who breastfeed (290 mcg RDA) following delivery because iodine is actively secreted into breast milk where it is critically important for infant nutrition.
Within the last year, the American Academy of Pediatrics joined the American Thyroid Association (ATA), The Endocrine Society and the Teratology Society in recommending daily 150-mcg iodine supplements for women who are planning a pregnancy, women who are pregnant or women who are lactating.6, 7, 8, 9 However, such supplements may be difficult to find. Only about half of the different types of prenatal multivitamins marketed in the U.S. contain iodine, and only 28% of prescription prenatal multivitamins contain iodine.10 Patients should be instructed to check labels or to ask their pharmacists about prenatal multivitamin iodine content.
Median urine iodine concentrations can be used to gauge the iodine status of populations. However, because of day-to-day and even hour-to-hour variability in urinary iodine content, it has been estimated that 10 24-hour or spot urine samples are needed in order to assess the iodine stratus of an individual with 20% precision.11
Spot or 24-hour urinary iodine concentrations can be used to confirm recent excess iodine exposure or to monitor compliance with a low-iodine diet. Nevertheless, at present, there are no validated individual biomarkers to reliably estimate chronic iodine status.
Too much, as well as too little, iodine in the diet can cause thyroid dysfunction in susceptible individuals.
Normally, in the setting of excess iodine ingestion the acute Wolff-Chaikoff effect transiently shuts down thyroid hormone synthesis. Following a few days of continued high iodine exposures, there is an “escape” from the acute Wolff-Chaikoff effect mediated by downregulation of the sodium iodine symporter, and normal thyroid hormone production resumes. Individuals with nodular autonomy are at particular risk for the development of iodine-induced hyperthyroidism (failure of the acute Wolff-Chaikoff effect).
Those with Hashimoto’s thyroiditis, a history of postpartum thyroiditis or a history of treated Graves’ disease are most susceptible to the development of iodine-induced hypothyroidism. During gestation, the fetus is also susceptible to the development of iodine-induced hypothyroidism in the setting of excessive maternal iodine intakes.
The tolerable upper limit for iodine is 1,100 mcg daily for adults. The ATA public health committee recently published a statement regarding the risks of excessive iodine intake, and specifically recommended against the use of supplements containing 500 mcg or more iodine daily.12
A recent study examined 10 different U.S. nutritional supplements marketed specifically for thyroid support and found that nine of them illegally contained thyroxine (T4) and/or triiodothyronine (T3).13 The iodine content of such supplements has not been measured. Patients are best advised to view these supplements with caution.
Elizabeth N. Pearce, MD, MSc, is an Endocrinology Advisor Editorial Board member. She is also Associate Professor of Medicine at the Boston University School of Medicine in the Section of Endocrinology, Diabetes and Nutrition.
- Bath SC, Steer CD, Golding J, Emmett P, Rayman MP. Effect of inadequate iodine status in UK pregnant women on cognitive outcomes in their children: results from the Avon Longitudinal Study of Parents and Children (ALSPAC). Lancet. 2013;382(9889):331-337.
- Hynes KL, Otahal P, Hay I, Burgess JR. Mild iodine deficiency during pregnancy is associated with reduced educational outcomes in the offspring: 9-year follow-up of the gestational iodine cohort. J Clin Endocrinol Metab. 2013;98(5):1954-1962.
- Caldwell KL, Pan Y, Mortensen ME, Makhmudov A, Merrill L, Moye J. Iodine status in pregnant women in the National Children’s Study and in U.S. women (15-44 years), National Health and Nutrition Examination Survey 2005-2010. Thyroid. 2013;23(8):927-937.
- Leung AM, Lamar A, He X, Braverman LE, Pearce EN. Iodine status and thyroid function of Boston-area vegetarians and vegans. J Clin Endocrinol Metab. 2011;96(8):E1303-1307.
- Pearce EN, Pino S, He X, Bazrafshan HR, Lee SL, Braverman LE. Sources of dietary iodine: bread, cows’ milk, and infant formula in the Boston area. J Clin Endocrinol Metab. 2004;89(7):3421-424.
- Council on Environmental Health, Rogan WJ, Paulson JA, Baum C, Brock-Utne AC, Brumberg HL, Campbell CC, Lanphear BP, Lowry JA, Osterhoudt KC, Sandel MT, Spanier A, Trasande L. Iodine deficiency, pollutant chemicals, and the thyroid: new information on an old problem. Pediatrics. 2014;133(6):1163-1166.
- Stagnaro-Green A, Abalovich M, Alexander E, Azizi F, Mestman J, Negro R, Nixon A, Pearce EN, Soldin OP, Sullivan S, Wiersinga W. Guidelines of the American Thyroid Association for the diagnosis and management of thyroid disease during pregnancy and postpartum. Thyroid. 2011;21(10):1081-1125.
- 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(8):2543-2565.
- Obican SG, Jahnke GD, Soldin OP, Scialli AR. Teratology public affairs committee position paper: iodine deficiency in pregnancy. Birth Defects Res A Clin Mol Teratol. 2012;94(9):677-682.
- Leung AM, Pearce EN, Braverman LE. Iodine content of prenatal multivitamins in the United States. N Engl J Med. 2009;360(9):939-940.
- König F, Andersson M, Hotz K, Aeberli I, Zimmermann MB. Ten repeat collections for urinary iodine from spot samples or 24-hour samples are needed to reliably estimate individual iodine status in women. J Nutr. 2011;141(11):2049-2054.
- Leung AM, Avram AM, Brenner AV, Duntas LH, Ehrenkranz J, Hennessey JV, Lee SL, Pearce EN, Roman SA, Stagnaro-Green A, Sturgis EM, Sundaram K, Thomas MJ, Wexler JA. Potential Risks of Excess Iodine Ingestion and Exposure: Statement by the American Thyroid Association Public Health Committee. Thyroid. 2014; doi:10.1089/thy.2014.0331.
- Kang GY, Parks JR, Fileta B, Chang A, Abdel-Rahim MM, Burch HB, Bernet VJ. Thyroxine and triiodothyronine content in commercially available thyroid health supplements. Thyroid. 2013;23(10):1233-1237.