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.
Vegans eschew all animal products, including dairy and eggs, so their iodine sources may be few.
WHO YOU ARE, WHERE YOU LIVE
The current findings are based on 256 healthy adults who had normal thyroid when they entered the study. Zhang's team, which did not respond to requests for comment, randomly assigned them to take one of 12 doses of supplemental iodine -- anywhere from 0 to 2,000 micrograms per day, for four weeks.
Of the people who took 400 micrograms, 5 percent developed subclinical hypothyroidism. And the numbers rose in tandem with the iodine dose: Of people on the highest dose (2,000 micrograms per day), 47 percent developed subclinical hypothyroidism.
"These are interesting data," Pearce said, "because we don't have a lot of information on iodine excess."
Subclinical hypothyroidism has no obvious symptoms, but studies have linked it to an increased risk of heart disease over the long term, Pearce noted.
Those studies don't prove that subclinical hypothyroidism is to blame. Still, they raise concerns that there could be health consequences.
But in general, Pearce said, it's thought that the effects of your iodine intake may depend on "who you are and where you live."
In certain parts of the world, the soil is low in iodine, and people who eat mainly local foods have a high risk of deficiency. In other parts of the world -- Japan, for example -- people have a high iodine intake starting early in life, and they seem to "tolerate" that high level, Pearce explained.
In China, natural iodine levels vary by region. The country introduced universal salt iodization in 1996, so the problem of iodine deficiency has been controlled in most areas.
But Pearce said it's not clear if the adults in this study had adequate iodine intake early in life. If not, that could be a factor in their response to iodine supplements.