Comparative epidemiologic study on the prevalence of iodine-induced hypothyroidism

Objective To investigate the prevalence of overt and subclinical hypothyroidism in the areas with different iodine intakes. Methods A cross sectional epidemiologic study in 3 rural communities of Panshan (iodine deficient), Zhangwu (covered by iodinated salt from 1996) and Huanghua (iodine excessive) was performed. In these communities, 4343, 7714 and 4230 inhabitants were respectively questioned about thyroid disorders. Of them 1103, 1584 and 1074 were physically examined and sera were sampled. The measurements included TSH, thyroid peroxidase antibody (TPOAb) and thyroglobulin antibody (TGAb) in the sera. The serum FT 4, FT 3 and TSH receptor antibody (TRAb) were determined for the subjects with abnormal TSH. Urinaryiodineconcentrationandthyroidultrasounic imaging were also examined. Results The median urinary iodine (MUI) was 103.1 μg/L in Panshan, 374.8 μg/L in Zhangwu and 614.6 μg/L in Huanghua. The prevalence of overt hypothyroidism was 0.27%, 0.95% and 2.05%in Panshan, Zhangwu and Huanghua, respectively. Overt hypothyroidism occured more frequently in the women over 45 years. The prevalence of subclinical hypothyroidism was 0.91%, 2.90% and 5.96% in Panshan, Zhangwu and Huanghua, respectively. The main cause of overt hypothyroidism was autoimmune thyroiditis; One third of the cases with subclinical hypothyroidism showed positive thyroid autoantibodies. Conclusion Higher prevalence of overt and subclinical hypothyroidism was found in the rural communities with higher iodine supply in a cross sectional epidemiologic study, suggesting a possible role of increasing or exessive iodine intake in hypothroidism.