WEDNESDAY, June 28 (HealthDay News) -- For decades, Americans have avoided iodine deficiency by consuming salt specially fortified with iodine. But a new study out of China suggests that, in some cases, fortification can lead to excessive iodine intake, which has its own health risks.

The trick, experts say, is to find a safe, mid-range dose of the trace element, which is essential to healthy thyroid function.

"That's the case with any mineral or vitamin -- there's a level below which you can end up with a deficiency, and there's a level above which you can consume an excess," said Roberta Anding, a registered dietitian and spokeswoman for the American Dietetic Association.

She and another expert said that, despite the results of the Chinese study, the major iodine-related problem facing Americans is still deficiency, especially among pregnant women and those people who cut most salt from their diet.

"I really couldn't find cases or case-reports of iodine overdose in the U.S.," Anding said.

That's not the situation in China, according to a study published in the June 29 issue of the New England Journal of Medicine. Researchers led by Dr. Weiping Teng, of China Medical University in Shenyang, compared iodine concentrations in urine samples from more than 3,000 people living in three different regions of the country.

They also examined the incidence of hypothyroidism (underactive thyroid) and autoimmune thyroiditis (also known as Hashimoto's disease) in these populations. Both of these conditions are linked to excessive iodine intake.

The team found that rising levels of iodine intake was closely linked to a rising incidence of the two thyroid conditions, which can cause fatigue and a general physiological slowdown.

While all of the people consumed iodized salt, some communities were much more affected than others.

"Those people were probably getting iodine [naturally] in the water, which others didn't, and then they were also getting the iodized salt," explained Dr. Robert Utiger, a clinical professor of medicine at Harvard Medical School, and the author of an editorial on the study in the journal. Together, that double-dose pushed people into an excess of daily iodine, he explained.

Utiger believes countries or communities that are considering iodine fortification of salt should first discern how much of the nutrient people are already obtaining via food and water.

But he stressed that iodine deficiency -- not excess -- remains the principal area of concern, both globally and in the United States. For example, despite the widespread use of iodized salt, the percentage of pregnant women with iodine deficiency "has increased in the U.S. over the past 20 years," Utiger said -- up from just 1 percent in the 1970s to 7 percent by 2002.

That's troubling, Utiger added, because a severe deficiency of the nutrient can lead to miscarriage or such serious birth defects as mental retardation. And even mild deficiencies in childhood have been linked to learning disabilities, poor growth and goiter in school-age children.

Why the upswing in iodine-deficient Americans? According to Anding, multiple factors may be at play.

"Of course, our richest source of iodine is iodized salt," she said. "But I see more and more people going back to more 'natural' versions -- non-iodized sea salt, for example -- or people cutting back on their added salt."

Indeed, cutting back on salt is healthy, in terms of avoiding high blood pressure and stroke. In fact, the American Medical Association earlier this month called on Americans to cut their daily sodium consumption in half over the next 10 years.

Anding agreed that too much salt is bad for people. But she said iodized table salt remains a prime source of iodine, so extreme cutbacks can be harmful. "I actually had a dietitian friend of mine who found out she had an overt iodine deficiency," Anding said. "That's because she's a good dietitian and didn't use a whole lot of salt," she added.

Another problem is that many food manufacturers are now using non-iodized salt, she said. Unlike many countries, the United States does not mandate that all commercial salt be iodized, although government studies show that about 70 percent of table salt does contain the nutrient.

Utiger said he'd like to see fewer American women with iodine deficiency, but the results of the Chinese study suggest that boosting concentrations of iodized salt comes with its own risks.

"There's a trade-off: If you raise the iodine intake of the population as a whole, you will reduce the number of pregnant women who have poor iodine intake," he said. "But a few more people may then have the effect of too much iodine," he noted.

More information

For more on iodine, head to Oregon State University.