Current Iodine Status in Japan: A Cross-sectional Nationwide Survey of Schoolchildren, 2014-2019

Japan has been regarded as a long-standing iodine-sufficient country without iodine fortification; however, data on nationwide iodine status are lacking.

This study aimed to characterize the iodine status in Japan.

From 2014 through 2019, a nationwide school-based survey was conducted across all districts in Japan. Urinary iodine concentration (UIC), creatinine (Cr) concentration, and anthropometry were assessed in healthy school-aged children (SAC) aged 6 to 12 years. Their iodine status is regarded as generally representative of the nation's iodine status.

A total of 32 025 children participated. The overall median UIC was 269 μg/L, which was within the World Health Organization's adequacy range. There was a regional difference in UIC values within 14 regions, and the lowest and highest median UICs were found in Tanegashima Island (209 μg/L) and Nakashibetsu, Hokkaido (1071 μg/L), respectively. The median UIC ≥ 300 μg/L was observed in 12 of 46 regions. By using estimated 24-hour urinary iodine excretion (UIE), the prevalence of SAC exceeding the upper tolerable limit of iodine for Japanese children was from 5.2% to 13.7%. The UIC values did not change with age, body surface area and body mass index percentile, whereas the Cr concentration simultaneously increased suggesting the effect of urinary creatinine on UI/Cr and estimated 24-hour UIE values.

The iodine intake of Japanese people is adequate, but in some areas it is excessive. The incidence and prevalence of thyroid disorders associated with iodine intake should be obtained, especially in the areas where high amounts of iodine are consumed.