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Too Much Iodine Can Increase Autoimmune Thyroiditis

Iodine intake linked to increases in hypothyroidism and autoimmune cases in China

WEDNESDAY, June 28 (HealthDay News) -- Too much iodine consumption has been linked to an increase in hypothyroidism and autoimmune thyroiditis cases in China, according to a study in the June 29 issue of the New England Journal of Medicine.

Weiping Teng, M.D., of China Medical University in Shengyang, China, and colleagues studied the effect of variations in iodine consumption in different regions of China on thyroid hormone levels in 3,761 participants in a 1999 study and 3,018 involved in a five-year follow-up.

The researchers found that 0.2 percent of those who were mildly deficient in iodine had hypothyroidism, as did 0.5 percent consuming more than enough iodine, and 0.3 percent taking excessive amounts of iodine. The corresponding rates for those who had subclinical hypothyroidism were 0.2 percent, 2.6 percent and 2.9 percent. For autoimmune thyroiditis, the corresponding rates were 0.2 percent, 1.0 percent and 1.3 percent.

Those with euthyroidism and antithyroid antibodies who took more than enough or excessive amounts of iodine were also more likely to have elevated serum thyrotropin levels after five years.

"More than adequate or excessive iodine intake may lead to hypothyroidism and autoimmune thyroiditis," the authors write.

An accompanying editorial states that "overall, the small risks of chronic iodine excess are outweighed by the substantial hazards of iodine deficiency, which is still widespread."

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