Thyroid Health: How Much Iodine Helps?

Chinese Study Shows Increased Iodine Connected to Decreased Thyroid Function

Medically Reviewed by Louise Chang, MD on June 28, 2006

June 28, 2006 -- A Chinese study links mild thyroid symptoms to too much iodine. But a Harvard expert says Americans need more iodine, not less.

Iodine is the key ingredient in the important hormones made by the thyroid gland in the throat. But your body doesn't make iodine -- you have to get some in your everyday diet.

Iodine deficiency is particularly dangerous for two groups. Pregnant women with severe iodine deficiency risk birth defects and mental retardation in their children. Children who get too little iodine have slow mental development.

That's why many nations require iodine to be added to table salt and salt for agricultural animals. While the U.S. does not mandate this, about 70% of American households regularly use iodized salt.

But what if you already get enough iodine? Can it hurt to get more?

Yes -- at least a little, find Weiping Teng, MD, and colleagues at China Medical University in Shengyang, China. The researchers compared iodine levels and ultrasound thyroid exams for people in areas of China with "mildly deficient," "more-than-adequate," and "excessive" levels of iodine intake.

They found that over time -- in a nation that mandates addition of iodine to salt -- increased iodine intake was linked to decreased thyroid function. The findings appear in the June 29 issue of The New England Journal of Medicine.

"Although iodine supplementation should be implemented to prevent and treat iodine-deficiency disorders, supplementation should be maintained at a safe level," Teng and colleagues conclude. "Levels that are more than adequate ... or excessive ... do not appear to be safe."

Expert: More Iodine Is Better

"Iodine Nutrition -- More Is Better," is the title of an editorial accompanying the Teng study. Editorialist Robert D. Utiger, MD, is a thyroid expert at Harvard Medical School and Brigham and Women's Hospital in Boston.

Utiger says that Teng and colleagues' conclusions are overcautious.

"Low iodine intake is more worrisome than the slight risk at the higher levels of intake," Utiger tells WebMD. "To eliminate iodine deficiency even in a small percentage of the population, the intake for the whole population has to be high. I am willing to pay the price of a few cases of mild symptoms at the high end to get a reduction in the very severe consequences at the low end."

Too much iodine does not worry nutritionist Leslie Bonci, MPH, RD, director of sports nutrition at the University of Pittsburgh Medical Center.

"Too much iodine is not a concern," Bonci tells WebMD. "Iodine is essential but do we really worry about people being iodine-deficient? No. The issue is not to eliminate iodine as we cut back on our salt consumption. But we don't need to frantically seek out iodine."

Utiger, however, says that pregnant women and children should err on the side of a bit too much iodine. He notes that 7% to 8% of Americans have moderate iodine deficiency -- up from 1% in the 1970s. He'd says he would like to see more iodine added to salt -- as some European nations have recently mandated -- but he isn't optimistic this will happen in the U.S.

"Many multiple-vitamin supplements do contain some iodine, so that would be a way to ensure high enough iodine levels," Utiger says. "But I was very surprised to learn that half or more of the vitamin supplements for pregnant women don't contain iodine."

Fish and dairy products are rich natural sources of iodine, so are some seaweeds, such as wakame.

Show Sources

SOURCES: Teng, W. The New England Journal of Medicine, June 29, 2006; vol 354: pp 2783-2793. Utiger, R.D. The New England Journal of Medicine, June 29, 2006; vol 354: pp 2819-2821. Robert D. Utiger, MD, clinical professor of medicine, Harvard University; senior physician, Brigham and Women's Hospital, Boston. Leslie Bonci, MPH, RD, director, sports nutrition, University of Pittsburgh Medical Center.
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