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The frequency of hyperthyroidism is not related to iodine intake
The background of the study. The frequency of hyperthyroidism may be increased in people whose iodine intake is high. In this study, the cumulative incidence of overt hyperthyroidism and the outcome of subclinical hyperthyroidism were determined in three communities in China in which iodine intake varied but had not changed in more than five years.
How the study was done. In 1999, 3671 people living in three communities in China completed questionnaires about thyroid disease and had measurements of serum thyrotropin (TSH), free thyroxine (T4), and urinary iodine. The same studies were done in 3108 of them (80 percent) in 2004. Overt hyperthyroidism was defined as low serum TSH and high free T4 concentrations, and subclinical hyperthyroidism as low serum TSH and normal free T4 concentrations.
The three communities were Panshan (median urinary iodine, 97 µg/L), Zhangwu (362 µg/L), and Huanghua (597 µg/L). The urinary iodine values in the three communities were very similar in 1999 (103, 374, and 615 µg/L, respectively). The people in Panshan made their salt from iodine-poor water. Zhangwu had been a region of iodine deficiency, but iodized salt was made available in 1996. In Huanghua, the water had long contained a high concentration of iodine. These communities were considered to have iodine deficiency, more than adequate iodine intake, and excessive iodine intake, respectively.
The results of the study. The mean age of the 3018 subjects was 43 years; there were 2316 women and 702 men. The five-year cumulative incidence of overt hyperthyroidism was similar in the three communities (Panshan, 1.4 percent; Zhangwu, 0.9 percent; and Huanghua, 0.8 percent).
Ninety-two of the 115 subjects (80 percent) who had subclinical hyperthyroidism in 1999 were studied in 2004. None received any antithyroid therapy in the interim. In 2004, 66 (72 percent) were normal, 18 (20 percent) still had subclinical hyperthyroidism, 5 (5 percent) had overt hyperthyroidism, and 3 (3 percent) had subclinical hypothyroidism. There were no differences in the changes in the three communities.
The conclusions of the study. The cumulative five-year incidence of overt hyperthyroidism is similar in people in different communities in which iodine intake varied from mild deficiency to excess.
The original article. Yang F, Shan Z, Teng X, Li Y, Guan H, Chong W, Teng D, Yu X, Fan C, Dai H, Yu Y, Yang R, Li J, Chen Y, Zhao D, Mao J, Teng W. Chronic iodine excess does not increase the incidence of hyperthyroidism: a prospective community-based epidemiological survey in China. Eur J Endocrinol 2007;156:403-8.
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| Thyroid Digest Index | | | July 2007 Thyroid Digest |
