Nationwide survey on iodine nutritional status and thyroid function in Japanese adults

Japan is an iodine-replete country without iodine fortification. To characterize the current iodine status and thyroid function in healthy adults, a nationwide cross-sectional observational survey was conducted between 2016 and 2023. Iodine concentration in urine (UIC), serum and scalp hair as well as serum TSH, FT4 and FT3 concentrations, thyroid antibody (ThAb), thyroid volume by sonography, dietary iodine intake by diet study and anthropometry were evaluated. A total of 2,845 adults with the mean age of 45.6 years were recruited from 20 regions across Japan. The median UIC (mUIC) was 295.0 μg/L and within the WHO's adequacy range of iodine intake. There was a regional difference in indices of iodine status, thyroid function and prevalence of ThAb positivity throughout all of the regions. The mUIC was lowest in Tanegashima Island (169 μg/L) and highest in Fukui (943 μg/L). The highest median TSH (2.11 μIU/mL) with the lowest FT4 (1.22 ng/dL) was observed in Ishikawa, Hokuriku while the lowest TSH (1.06 μIU/mL) was in Rebun Island, Hokkaido although the median values of TSH, FT4 and FT3 were within their reference range. The prevalence of ThAb positivity was 12.6%. A significant positive correlation was observed only between mUIC and median TSH. Japanese adults consume more iodine than the average intake of other ethnic groups and there were some areas with high iodine intake. Current average iodine intake was estimated to be 400 μg per day in this study population. A large-scaled epidemiological survey is required in future.