How much iodine is too much?

Iodine deficiency is a major health problem worldwide, but a new study points to the potential downsides of too much iodine.

Iodine is a mineral found in iodized salt, seafood, eggs, dairy and some breads. It is used by the thyroid gland to help regulate metabolism and development, especially in babies and children.

Iodine deficiency during fetal and early-childhood development is a leading cause of brain impairments in much of the world. So most research has been directed at the effects of inadequate iodine.

Less is known about how much iodine is too much. So for the new study, reported in the American Journal of Clinical Nutrition, Chinese researchers randomly assigned healthy adults to take various doses of iodine supplements for four weeks.

They found that at relatively higher doses - 400 micrograms a day and up - study participants began developing what's called subclinical hypothyroidism.

That refers to a dip in the body's thyroid hormone levels, but with no obvious symptoms of hypothyroidism - which include problems like fatigue, depression, dry skin and weight gain.

In this study, people taking 400-microgram supplements were getting around 800 micrograms of iodine per day when diet was factored in.

So the findings suggest that people - at least in China - should get no more than 800 micrograms a day, according to the researchers, led by Wanqi Zhang of Tianjin Medical University.

That's different from what's recommended in the U.S., where National Institutes of Health guidelines say the safe upper limit for adults is 1,100 micrograms of iodine per day.

Still, the typical American would get much less than 800 micrograms of iodine a day through diet anyway, according to Dr. Elizabeth Pearce, an associate professor of medicine at Boston University who was not involved in the study.

That said, Pearce cautioned against taking iodine supplements with more than 150 micrograms in a daily dose. And most Americans could skip supplements altogether.

"Overall, we're iodine-sufficient," said Pearce, who studies iodine sufficiency and thyroid function.

But she said there are certain people who may need supplements, including pregnant women.

In the U.S., adults are advised to get 150 micrograms of iodine each day; pregnant women should get 220 micrograms, while breastfeeding moms are told to get 290 micrograms.

The American Thyroid Association recommends that pregnant and breastfeeding women take a vitamin with iodine because low iodine can increase the risk of miscarriage and thyroid problems in moms, in addition to mental disabilities in babies.

According to Pearce, vegans may also want to take a supplement. In a recent study, Pearce and her colleagues found that the average iodine level in a group of 63 vegans was lower than what's recommended - though their thyroid hormone levels were in the normal range.