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Increasing iodine intake is associated with higher rates of hypothyroidism
The
background of the study. Both iodine deficiency and iodine excess
cause thyroid dysfunction. In this study, thyroid function was assessed
in 1999 and again in 2004 in residents of regions in China in which iodine
intake was different and did not change.
How the study was done. In 1999, 3761 people living in three regions in China were examined, underwent thyroid ultrasonography, and had measurements of serum thyrotropin (TSH), thyroxine (T4), antithyroid peroxidase (anti-TPO) antibodies, and antithyroglobulin (anti-Tg) antibodies, and urinary iodine. The same studies were done in 3018 of these people in 2004. The regions were Panshan (mild iodine deficiency), Zhangwu (adequate iodine intake), and Huanghua (high iodine intake).
The results of the study. The ratio of women to men was approximately 3:1 in each region. The median urinary iodine excretion in 1999 and 2004 was, respectively, 103 and 97 µg/L in Panshan, 375 and 350 µg/L in Zhangwu, and 615 and 635 µg/L in Huanghua.
The prevalence and cumulative incidence of overt hypothyroidism and subclinical hypothyroidism increased with increasing iodine intake. The increase in cumulative incidence of hypothyroidism was especially prominent among people with high serum anti-TPO or anti-Tg antibody values at base line. The prevalence and cumulative incidence of diffuse goiter decreased with increasing iodine intake, as did the cumulative incidence of nodular goiter. There were no differences in the rates of Graves’ disease in the three regions at either time.
The conclusions of the study. Hypothyroidism and autoimmune thyroiditis are more common among people who have a high iodine intake.
The original article. Teng W, Shan Z, Teng X, Guan H, Li Y, Teng D, Jin Y, Yu X, Fan C, Chong W, Yang F, Dai H, Yu Y, Li J, Chen Y, Zhao D, Shi X, Hu F, Mao J, Gu X, Yang R, Tong Y, Wang W, Gao T, Li C. Effect of iodine intake on thyroid diseases in China. N Engl J Med 2006;354:2783-93.
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