[Screening for thyroid dysfunction in adults residing in Hokkaido Japan: in relation to urinary iodide concentration...

The prevalence of thyroid dysfunction and its relation to thyroid autoantibodies (TAA) and urinary iodide concentration (UI) was studied in apparently healthy adults in Sapporo (n = 4110) (Sapporo group), and in five coastal areas of Hokkaido (n = 1061) (coastal group) which produce iodine-rich seaweed (kelp). The frequency of above normal UI (high UI) in the morning urinary samples of coastal group was 10.8%, significantly higher than that of Sapporo group (6.4%) (p < 0.001). Frequency of positive TAA in both groups were similar. In Sapporo group TAA was positive in 6.4% of males and 13.8% of females with an age-related increase. The overall prevalence of hyperthyroidism (TSH < 0.15 mU/L) in coastal group (0.6%) was similar to that in Sapporo group (1.1%), while that of hypothyroidism (TSH > 5.0 mU/L) in coastal group (3.8%) was significantly higher than that in Sapporo group (1.3%) (P < 0.001). The frequency of high UI correlated significantly with that of hypothyroidism with negative TAA (r = 0.829, P < 0.05), but not with positive TAA, or with that of hyperthyroidism. Hypothyroidism was more prevalent in TAA negative subjects with high UI than with normal UI. Moreover, serum TSH and thyroglobulin levels were higher and free T4 level was lower in former than in latter group. These results indicate that 1) the prevalence of TAA negative hypothyroidism in iodine sufficient areas may be associated with the amount of iodine ingested, 2) this hypothyroidism is more prevalent and marked in subjects consuming further excess amounts of iodine, and 3) excessive intake of iodine should be considered an etiology of hypothyroidism in addition to chronic thyroiditis in these areas.