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BOSTON, Feb. 25 -- Prenatal multivitamins that use potassium iodide as the iodine source give a more consistent dose of the element than those that use kelp, researchers here said.

Multivitamins that included potassium iodide consistently contained about 75% of the iodine content listed on their labels, whereas others that used kelp had large variations of the element, Elizabeth Pearce, M.D., of Boston University Medical Center, and colleagues reported in the Feb. 25 issue of the New England Journal of Medicine.

Although the findings favor potassium iodide over kelp, Dr. Pearce said the bigger problem is the lack of vitamins on the market that contain any amount of the element.

"Many of the multivitamins don't contain any iodine at all, which is the most troubling thing," she said. "Our major concern is that it's just not there."

Iodine deficiency in mothers can have adverse effects on fetuses and breast-fed infants, as it is essential for normal thyroid function and neurocognitive development.

But iodine intake in the U.S. has decreased by about 50% since the 1970s, the researchers said.

Dr. Pearce said this could be due to decreased consumption of iodized salt over fears of hypertension.

Several groups have made recommendations about appropriate iodine consumption for pregnant women and nursing mothers. The Institute of medicine recommends a daily 220 mcg during pregnancy and 290 mcg during lactation, the World Health Organization suggests 250 mcg a day for both groups, and the American Thyroid Association recommends 150 mcg daily.

To determine the dosages that pregnant women actually receive, the researchers identified 114 prenatal multivitamins that contained iodine. Of these, labels on 101 of them indicated that they contained 150 mcg or more of iodine per dose.

For 59% the iodine was in the form of potassium iodide; another 37% used kelp.

The researchers measured the iodine content of 60 of the multivitamins and found that the mean level of measured iodine typically varied from the labeled dose.

For the vitamins containing potassium iodide, the mean measured level was 119 mcg per dose; those containing kelp ranged from 33 mcg to 610 mcg per dose.

A total of 13 brands contained levels of iodine that were discordant by 50% or more with the values on their labels.

The researchers said that potassium iodide contains 76% iodide, which means the measured content of iodine was equal to about 76% of the total potassium iodide content.

"Manufacturers of prenatal multivitamins in the U.S. should be encouraged to use only potassium iodide, to maintain consistency in labeling, and to ensure that these vitamins contain 150 mcg of supplemental daily iodine," the researchers said.

Dr. Pearce said it was a measure of "simple math" for manufacturers to include 200 mcg of potassium iodide per dose to be sure pregnant women get the recommended 150 mcg of iodine daily.

She also noted that the 610 mcg dose of iodine in some of the multivitamins using kelp is "probably too much," although there is only one recommendation on a limit to dietary iodine. It's from the World Health Organization and it's a "conservative guess that's not based on much evidence."

Still, she said the "most important point" of the study was to make women and their healthcare providers aware that "iodine is critical in pregnancy and lactation and they should be looking for a prenatal vitamin that has iodine in it, preferably in the form of potassium iodide."

The researchers reported no disclosures.

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