The Effect of Iron and Iodine on the Association Between Particular Matter 2.5 and Thyroid Function in the First Trimester of Pregnancy.

The effect of particulate matter 2.5 (PM) on thyroid function has been reported. However, thyroid function can be affected by iron and iodine, which are essential for the synthesis of thyroid hormones. This study aimed to investigate the relationship between PM exposure and thyroid function during the first trimester (T1) and evaluate whether iron or iodine status mediate this association. Demographic information, serum ferritin (SF), and thyroid function, including free thyroxine (FT) and thyroid-stimulating hormone (TSH), were collected from 3922 pregnant women at Peking University First Hospital between October 2017 to August 2018. Serum iodine (SI) levels were additionally measured in women with hypothyroxinemia. Ambient PM concentrations were estimated by monitor stations. The general linear models were employed to assess the association between PM exposure and thyroid function. Furthermore, mediation analysis was performed to evaluate the potential mediating roles of iron status in the relationship between PM exposure and thyroid function. In the first trimester, per 1-interquartile range (IQR) increase in PM exposure was associated with a significant decrease in maternal FT levels (β = - 0.166; 95% CI - 0.326, - 0.006; P = 0.042), but showed no significant association with TSH levels. Furthermore, each IQR increase in PM exposure was associated with a 48.3% higher risk of hypothyroxinemia (OR = 1.483; 95% CI 1.111, 1.982; P = 0.008). Among women with hypothyroxinemia, the median SF and SI levels were 82.93 μg/L and 79.95 μg/L, respectively. Mediation analysis within a subgroup of 2915 women with available SF data revealed a non-significant indirect effect of SF on the association between PM exposure and FT levels (β = - 0.008; 95% CI - 0.027, 0.001; P > 0.05). Our findings demonstrate that first-trimester exposure to PM was associated with a slight decrease of FT levels, but in the absence of significant changes in TSH, and an increased risk of hypothyroxinemia. Furthermore, these associations may not be mediated by maternal iron or iodine status.