Thyroid Function in Japanese Adults as Assessed by a General Health Checkup System in Relation with Thyroid-Related Antibodies and Other Clinical Parameters

Background: A limited number of epidemiological studies have attempted to assess thyroid function in the general population of iodine-sufficient countries. The aim of the present study was to determine the underlying thyroid diseases responsible for abnormal thyroid function detected by a general health checkup system in Japan, and to characterize the lipid metabolism in subjects found to have thyroid dysfunction. Methods: Serum thyrotropin (TSH), free thyroxine, anti-thyroglobulin antibodies (TgAb), anti-thyroid peroxidase antibodies (TPOAb), and TSH-binding inhibitor immunoglobulins (TBII) were determined in 1818 Japanese adults (804 men and 1014 women; mean age 51.3 +/- 9.0 years) who undertook a general health checkup. Results: Of the 1818 examinees, 12 (0.7%) had overt hypothyroidism (OH), 105 (5.8%) subclinical hypothyroidism, 13 (0.7%) overt thyrotoxicosis, and 39 (2.1%) subclinical thyrotoxicosis. TgAb or TPOAb tests were positive in 17.7% of men and 31.4% of women. The prevalence of positive tests for TgAb or TPOAb was 14.8% for men and 23.4% for women without palpable goiter. Positive tests for TgAb, TPOAb, TBII, and a palpable goiter were more common in subjects with abnormal thyroid function tests than in subjects with normal thyroid function. At the time that abnormal thyroid function test results were first obtained, the signs of thyrotoxicosis were mild or even absent in all 13 subjects with overt thyrotoxicosis, 8 of whom had Graves' disease and 5 of whom had painless thyroiditis. Of the 12 patients with OH, only 2 patients had a palpable goiter. In the OH group, TgAb tests were positive in eight, TPOAb tests were positive in eight, and TBII tests were positive in two. The prevalence of disturbed lipid metabolism, when adjusted for age, was significantly higher in the subclinical hypothyroidism group than in normal controls (p < 0.001; odds ratio, 1.67; 95% confidence interval, 1.10-2.51). Conclusions: In Japanese adults who chose to be screened by a general health checkup system, the prevalence of abnormal thyroid function was nearly 10%. In a high percentage of these patients, abnormal thyroid function could not be detected by their history or physical examination. Just a physical examination without thyroid function tests, particularly serum TSH levels, was not adequate even when performed by a thyroid specialist.