Most US prenatal multivitamin brands appear to lack iodine or the actual dose of iodine they contain does not match values on the labeling, according to a letter posted in the February 26 issue of the New England Journal of Medicine.

“Iodine nutrition is critically important in pregnancy. Women who are deficient in pregnancy have children often with lower IQs or neurocognitive delays. Iodine deficiency is the leading cause of preventable mental retardation in the world,” explained Dr. Elizabeth Pearce, co-author of the letter and an assistant professor of medicine at Boston University Medical Center.

Pearce and colleagues tested the iodine content of 60 multivitamins with iodine and found that only 28 percent contained the full amount stated on the label.

According to the American Thyroid Association, pregnant and lactating women should take 150 micrograms per day for proper thyroid function. Having enough iodine during pregnancy is important for fetal brain development. Among the products tested for iodine which claimed to deliver that amount, one third had less than half that amount.

“The measured iodine content of multivitamins with kelp as the iodine source was extremely variable and often did not match labeled values. Prenatal multivitamins containing potassium iodine were a more reliable source,” Dr. Pearce said in a statement issued by the American Thyroid Association.

Thirty-eight percent of the world’s population is affected by iodine deficiency, which is the leading cause of preventable mental retardation. In developed countries, iodine is added to table salt. On contrary, in richer countries, iodine levels in food have been declining because salt is avoided for its damaging effect on blood pressure.