Only Half of US Prenatal Multivitamins Contain Iodine; Doses Often at Odds With Label

Susan Jeffrey

February 25, 2009

February 25, 2009 — Despite its being critical to normal neurocognitive development, a new study finds that only 51% of US prenatal multivitamin brands contain any iodine, and in a number of randomly selected brands, the actual dose of iodine contained in the supplements did not match values on the labeling.

The new report appears as correspondence in the February 26 issue of the New England Journal of Medicine.

Iodine Content Not Mandated

Normal thyroid function in fetuses and breast-fed infants is crucial for normal neurocognitive development and depends on sufficient dietary intake of iodine, the researchers, with corresponding author Elizabeth N Pearce, MD, from Boston University Medical Center, in Massachusetts, write.

Iodine deficiency affects more than 2.2 billion persons, or 38% of the world's population, and is the leading cause of preventable mental retardation. In the United States, National Health and Nutrition Examination Survey (NHANES) data suggest that iodine intake among women of child-bearing age has decreased by more than half, and a subset may have mild iodine deficiency. "Even mild iodine deficiency may have adverse effects on the cognitive function of children," the authors write.

The Institute of Medicine recommends a daily iodine intake of 220 µg during pregnancy and 290 µg during lactation. For the World Health Organization, those recommended values are 250 µg for both pregnant and lactating women.

The American Thyroid Association has recommended that women receive prenatal vitamins containing 150 µg of iodine daily for both pregnant and lactating women, the authors write. "However, the iodine content of prenatal multivitamins is not mandated in the United States."

Using the Internet, they identified 223 brands of prenatal multivitamins currently marketed in the United States, 127 prescription and 96 nonprescription brands. Only 114 contained iodine: 87 of the nonprescription and 27 of the prescription brands.

Of these, 101, or 89%, contained 150 µg or more of iodine per daily dose, at least according to their labels. However, that was not what the researchers found when they actually measured iodine content.

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

The mean level of measured iodine per daily dose in 35 brands containing potassium iodide as the iodine source was 119.0 + 13.6 µg. "Potassium iodide contains 76% iodide," they note. "Thus, the measured iodine content was approximately equivalent to 76% of the total potassium-iodide content."

Conversely, the measured iodine levels in 25 brands using kelp as the iodine source ranged from 33 µg to 610 µg per daily dose. There were 13 brands that contained levels that were discordant by 50% or more with the values on the label; 10 of these had levels that were lower by 50% or more.

"Manufacturers of prenatal multivitamins in the United States should be encouraged to use only potassium iodide, to maintain consistency in labeling, and to ensure that these vitamins contain 150 µg of supplemental daily iodine by including at least 197 µg of potassium iodide per daily dose, as recommended by the American Thyroid Association," the authors conclude.

N Engl J Med. 2009;360:939-940.

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