Objective: Despite the relatively high prevalence of thyroid disease, there are a few medical examination centers that test thyroid function. Recently, the Japan Thyroid Association has started treating subclinical hypothyroidism as a major clinical issue. In the present study, we investigated the prevalence and characteristics of thyroid dysfunction by means of serum TSH measurements. Methods: The subjects were 10,330 men and 6,334 women who underwent health check-ups in 2005. The mean ages

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... e 52.2 for males and 51.5 for females. Serum TSH concentrations were measured using a chemiluminescent immunoassay. Results: The ranges of TSH concentrations in males, females and overall were, respectively, 0.35~3.31, 0.41~4.16 and 0.36~3.67μU/ml. The prevalence of low TSH(<0.1μU/ml) was 0.5% for males and 1.0% for females, and decreased with age. The prevalences of high TSH (≧5.0μU/ml) were, respectively, 1.3% and 1.7%, and increased with age. Based on the results for subjects who underwent detailed examinations, we estimated the prevalence of thyroid diseases. The prevalences for both Graves'disease and silent thyroiditis were 0.19% for males and 0.45% for females. That of overt hypothyroidism was 0.56% for males and 0.55% for females and the respective prevalences of subclinical hypothyroidism were 0.41% and 0.69%. In subjects with high TSH levels, the prevalences for Hashimoto's thyroiditis were 38.5% and 61.1%, respectively. In a quarter of the subjects with high TSH levels, they had become normal at re-examination. Conclusion: The prevalences of thyroid dysfunction requiring detailed examination as defined by low(<0.1) and high ( ≧5.0) TSH levels were 1.8% for males and 2.7% for females. This suggests that the measurement of serum TSH concentrations would be useful in ningen dock.