Thyroid disorders in the general population of Hisayama Japan, with special reference to prevalence and sex differences

Serum triiodothyronine (T3), thyroxine (T4), thyroid stimulating hormone (TSH) and thyroid autoantibodies (TAA) to either microsome or thyroglobulin were measured in 1251 samples from the general population, 546 males and 705 females, aged 40 or over in Hisayama, a Japanese rural town. TAA was positive in 7.7% of men and 15.0% of women, and the male/female ratio was 1:2. This ratio was markedly different from those in hospital patients with thyroid disorders where ratios such as 1:8 for chronic thyroiditis and 1:4 for Graves' disease were found. Among the study subjects, definite hyperthyroidism was found in 0.2%, borderline hyperthyroidism in 0.7%, overt hypothyroidism in 0.3%, latent hypothyroidism in 4.2%, and non-thyroid illness in 2.4%. Overt thyroid dysfunction was more evident in women, while latent thyroid dysfunction or non-thyroidal illness was equally prevalent in men and women. These results indicate that the incidence of immunologically or functionally latent thyroid disorders is rather higher than expected, in both women and men. Some unknown factor(s) might act as a trigger to manifest thyroid dysfunction, mainly in females.