Iodine status-dependent associations between urinary trace elements and thyroid hormones in older women: Findings from KoGES

Iodine is central to thyroid hormone synthesis and may modify how other trace elements influence thyroid physiology, yet few studies have examined these relationships by iodine status.

To examine iodine status-stratified associations between urinary trace elements and thyroid function in older women.

We analyzed 652 women aged ≥ 60 years in the Korean Genome and Epidemiology Study. Nineteen trace elements were quantified in spot urine by inductively coupled plasma-mass spectrometry (mercury by direct mercury analyzer); urinary iodine and serum thyroid measures were cohort-derived. Associations were examined by multiple linear regression, with a trace element × iodine status interaction term and within iodine strata, and by quantile-based g-computation for mixtures, with false discovery rate (FDR) correction across elements.

Iodine status was insufficient in 44.6%, adequate in 30.7% and excessive in 24.7%; thyroid dysfunction (10.1%) and TPO antibody positivity (9.0%) did not differ across categories. Only zinc was associated with free T4 overall after FDR correction. Interaction tests and stratified models converged: among iodine-adequate women, Cu, Zn, Cd and Cs were positively associated with free T4 and inversely with TSH, whereas these associations were weak or absent in the other strata. The mixture was positively associated with free T4 only in that group. No element was consistently associated with thyroid autoimmunity.

Trace element-thyroid hormone associations were largely confined to women with adequate iodine status, consistent with effect modification by iodine. Accounting for iodine status may be needed to detect environmental influences on thyroid function.