Abstract Background The association of iodine nutritional status with thyroid function in iodine-replete countries is controversial and more sensitive biomarkers than urinary iodine concentration is required. Objective To evaluate the use of scalp hair and serum iodine concentration as measure of iodine status through their association with thyroid indices Methods A nationwide survey was conducted between 2016-2023. Iodine concentration in serum (SIC), hair (HIC), urine (UIC) and estimated 24-h urinary iodine excretion (eUIE), dietary iodine intake (DII) as well as serum TSH, FT4 and FT3 concentrations, thyroid antibody (ThAb) was measured in Japanese adults. Results The median UIC in 2,771 adults was 295.0 μg/L and within the WHO’s adequacy range of iodine intake. The high iodine intake assessed by UIC, eUIE and SIC was associated with higher TSH and lower thyroid hormone levels. UIC, eUIE and DII were higher in the group with a high TSH level. The subjects with positive ThAbs had significantly higher SIC, DII, TSH and slightly lower FT4 levels. The median HIC correlated positively but weakly with SIC, eUIE and DII, while SIC correlated modestly with eUIE, UIC and weakly with DII. The 5th percentile of the SIC reference (49.9 μg/L) coincided with the threshold value of median UIC (100 μg/L) for iodine deficiency by WHO. Conclusion Higher iodine intake results in changes of TSH and thyroid hormones within their referenced range. Serum and hair iodine concentrations might be feasible indicators for a population's iodine intake and further research is needed to validate their availability.
Fuse et al. (Thu,) studied this question.