Mediation effects of thyroid hormones in the associations between essential metal elements and anxiety or depression symptoms in the elderly.

To investigate essential metals' association with anxiety/depression symptoms in the elderly, and explore serum thyroid hormones' potential mediating role in this association. Study included 416 Shenzhen elderly. The Hamilton Anxiety Scale (HAMA) and the Geriatric Depression Scale (GDS) were utilized to evaluate symptoms of anxiety and depression. Essential metals in urine were measured using inductively coupled plasma mass spectrometry (ICP-MS), while thyroid hormone information was sourced from medical records. Mediation analysis assessed serum thyroid hormones' mediating effect in the link between essential metals and anxiety/depression. Urinary zinc (Zn) level was negatively linked to the risk of anxiety symptoms [odds ratio (OR) = 0.308, 95% confidence interval (CI): 0.133, 0.665; third vs. first tertile]. The urinary iron (Fe), Zn and selenium (Se) levels were negatively correlated with the risk of depressive symptoms (Fe: OR = 0.367, 95% CI: 0.173, 0.742; Zn: OR = 0.353, 95% CI: 0.160, 0.734; Se: OR = 0.488, 95% CI: 0.244, 0.949; all third vs. first tertile). The Bayesian kernel machine regression analysis (BKMR) revealed a negative relationship between the mixed exposure to essential metals and symptoms of anxiety and depression. Mediation analysis showed that total thyroxine (TT4) did not significantly mediate the link between urinary copper (Cu) levels and anxiety, with a mediation effect size of 0.020 (95% CI: -0.053, 0.000) and an estimated mediation proportion of 41.95%. Zn was negatively associated with anxiety, while Fe, Zn, and Se were negatively associated with depression. The mediation effect of TT4 on the association between urinary Cu and anxiety was not statistically significant, although the estimated mediation proportion was 41.95%.