Associations between polycyclic aromatic hydrocarbons exposure and maternal thyroid hormones during pregnancy

Polycyclic aromatic hydrocarbons (PAHs) are well-recognized endocrine-disrupting compounds. However, the association between prenatal PAH exposure and maternal thyroid hormones remains contentious. To resolve this uncertainty, we used data from 1165 pregnant women in the Zunyi Birth Cohort, aiming to characterize associations between urinary PAH metabolites and maternal thyroid hormone levels across gestation. We used linear mixed-effects models (LMMs) to examine associations between PAH metabolites and maternal thyroid hormones, restricted cubic spline (RCS) analysis to explore dose-response relationships, and Bayesian kernel-machine regression (BKMR) to assess the collective effects of PAH metabolites on the maternal thyroid system. We found that 2-OHNAP, 9-OHFLU, 4-OHPH, 3-OHPH, 1-OHPH, and 1-OHPYR were associated with thyroid-stimulating hormone (TSH), total triiodothyronine (TT3), total thyroxine (TT4), free triiodothyronine (FT3), and free thyroxine (FT4). Additionally, significant dose-response relationships were observed between 9-OHFLU, 2-OHFLU, 3-OHPH, 1-OHPH, 2-OHPH, 1-OHPYR and TSH, TT3, TT4, FT3, FT4. Notably, BKMR analysis revealed an overall non-linear, inverted U-shaped association between PAH metabolite mixtures and TSH, TT3, TT4, FT3, and FT4. Together, these results demonstrate that prenatal PAH exposure is associated with altered maternal thyroid hormone secretion. They further highlight the need to reduce PAH exposure during pregnancy and closely monitor maternal thyroid hormone levels throughout gestation.