The American Thyroid Association has recommended that all pregnant and breastfeeding women in the U.S. should take daily supplements containing 150 mcg iodine. However, a study conducted by researchers at Boston University Medical Center has found that only 51 percent of U.S. prenatal multivitamins contain iodine.

“Normal thyroid function in fetuses and breast-fed infants, which is dependent on sufficient intake of iodine, is crucial for a child’s normal neurocognitive development,” said Elizabeth N. Pearce, MD, assistant professor of medicine, in a research letter appearing in the February 26, 2009 issue of the New England Journal of Medicine (Vol. 360, No. 9).

According to the researchers, iodine deficiency affects more than 2.2 billion people worldwide and is the leading cause of preventable mental retardation. Over the last three decades, the iodine intake of U.S. women of childbearing age has decreased by more than half, and a subset of U.S. women of childbearing age may have mild iodine deficiency.

“Even mild iodine deficiency may have adverse effects on the cognitive function of children,” said Dr. Pearce. “The measured iodine content of multivitamins with kelp as the iodine source was extremely variable, and often did not match labeled values,” said Dr. Pearce. “Prenatal multivitamins containing potassium iodine were a more reliable source.”

The iodine content of prenatal vitamins is not mandated in the U.S., noted the researchers, who suggest that manufacturers of prenatal vitamins in the U.S. should be encouraged to ensure that their products contain the amount of iodine recommended by the American Thyroid Association and to use only potassium iodine – which contains 76 percent iodine – to maintain consistency in iodine content (Newswise).