Most prenatal multivitamin supplements contain less iodine than stated on the label, which could put newborns at risk for poor brain development, U.S. researchers say.

In a letter appearing in Thursday's New England Journal of Medicine, Dr. Elizabeth Pearce of Boston University School of Medicine and her colleagues searched the internet to find 127 nonprescription and 96 prescription prenatal multivitamins sold in the U.S.

The researchers then tested the contents, and found 60 brands, which were not disclosed, contained less iodine than indicated.

Vitamins that used kelp as the source of iodine had large variations in iodine content, compared with vitamins that got iodine from salt, the researchers found.

The quantity of iodine measured in supplements containing potassium iodide was about 75 per cent of that stated on the labels, according to the study.

Potassium iodide favoured

"The American Thyroid Association has recommended that women receive prenatal vitamins containing 150 micrograms of iodine daily during pregnancy and lactation. However, the iodine content of prenatal vitamins is not mandated in the United States," said Pearce.

"In order to maintain consistency of labelling and to ensure these vitamins contain the recommended dosage, we strongly propose that the manufacturers of these products use only potassium iodide at a dose of 200 micrograms per serving."

The U.S. Food and Drug Administration should mandate that all prenatal multivitamins contain iodine, agreed Dr. Alex Stagnaro-Green, who specializes in pregnancy thyroid problems at the Touro University College of Medicine in New Jersey.

Lack of iodine affects more than 38 per cent of the world's population, but there is no way to tell who is iodine deficient, the researchers said.

Mild iodine deficiency may harm the cognitive function of children, the study's authors noted.

Consumers should look for prenatal multivitamins with potassium iodide as the source since it is less variable, Pearce suggested.

Independent groups such as the United States Pharmacopeia test dietary supplements to verify contents, which are indicated with a seal of approval from USP.

Iodine is essential to the proper functioning of the thyroid. In some parts of the world, including Canada and the U.S., iodine is added to most salt and bread.

With files from the Associated Press