Scalp hair and serum iodine as potential biomarkers for iodine intake and their association with the thyroid function

Context: 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 and 2023. Iodine concentration in serum (SIC), hair (HIC), urine (UIC) and estimated 24-hour 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 2771 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. Urinary iodine concentration, 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.