Salt has been iodized in China since 1996, resulting in an increase in iodine intake throughout the country. In a 1999 study, researchers observed an increase in the prevalence of autoimmune thyroiditis, overt hypothyroidism, and subclinical hypothyroidism with increasing iodine intake in cohorts from three regions of China with different levels of iodine intake: "mildly deficient" (median urinary iodine excretion, 84 mcg/L), "more than adequate" (median, 243 mcg/L), and "excessive" (median, 651 mcg/L). Of the 3,761 subjects enrolled in the original study, 3,018 (80.2%) participated in a five-year follow-up study. During the follow-up period, among subjects with mildly deficient iodine intake, more than adequate intake, and excessive intake, the cumulative incidence of autoimmune thyroiditis was 0.2%, 1.0%, and 1.3%, respectively; that of subclinical hypothyroidism, 0.2%, 2.6%, and 2.9%, respectively; and that of overt hypothyroidism, 0.2%, 0.5%, and 0.3%, respectively. The differences in incidence for mildly deficient vs. more than adequate or excessive intake were statistically significant for autoimmune thyroiditis (p = 0.01 to 0.03) and for subclinical hypothyroidism (p < 0.001). The authors concluded that more than adequate or excessive iodine intake may lead to autoimmune thyroiditis and hypothyroidism.

Comment: Iodine deficiency remains an important problem in some parts of the world, and iodine intake should be increased in people whose intake is inadequate. High-dose iodine therapy also has a role in clinical medicine, particularly in the treatment of fibrocystic breast disease. However, people taking large amounts of iodine should be monitored for the development of thyroid abnormalities.

Teng W, et al. Effect of iodine intake on thyroid diseases in China. N Engl J Med. 2006;354:2783-2793.