Excess iodine increases thyroid disease risk
Dietary iodine supplementation that is more than adequate or excessive does not appear to be safe, especially for susceptible populations who either have iodine deficiency or potential autoimmune thyroid diseases, researchers reporting in the New England Journal of Medicine say.
"Our results suggest a link between increased iodine intake and hypothyroidism," the scientists, from the China Medical University in Shengyang, comment.
They make this statement following results of their 5-year follow-up study in which they investigated the effect of iodine-induced thyroid diseases in three regions of China with different levels of iodine intake. All salt has been iodized in China since 1996.
In total, data from 3018 individuals were assessed – 884 participants had mildly deficient iodine intake, 1270 had more than adequate iodine intake, and 864 had excessive iodine intake. The three groups were defined by median urinary iodine excretions of 84, 243, and 651 μg/l, respectively.
The cumulative incidence of overt hypothyroidism among those with mildly deficient, adequate or excessive iodine intake was roughly similar, at 0.2%, 0.5%, and 0.3%, respectively.
The rates of subclinical hypothyroidism, however, were significantly higher among those in areas of adequate and excessive iodine intake, at 2.6%, and 2.9%, respectively, compared with the rate among participants living in an area of mildly deficient iodine intake, at 0.2%.
Lead author Weiping Teng and colleagues continue that autoimmune thyroiditis was more prevalent among individuals who had adequate or excessive iodine intake, at respective rates of 1.0% and 1.3% versus 0.2% among those with mildly deficient iodine intake.
"Although iodine supplementation should be implemented to prevent and treat iodine-deficiency disorders, supplementation should be maintained at a safe level," the investigators advise.
"Supplementation programs should be tailored to the particular region," they conclude.
In an accompanying editorial, Robert Utiger, from the Brigham and Women's Hospital in Boston, Massachusetts, USA, cautions that, overall, the small risks of chronic iodine excess are outweighed by the substantial hazards of iodine deficiency, which is still widespread.
"All these hazards — pregnancy loss, goiter, and mental retardation — are prevented by an adequate iodine intake. To achieve this, iodine intake should be increased in most countries, not only in those with iodine deficiency but also in those whose iodine intake is marginal, so the iodine intake is at least 300 to 400 μg daily," he says.