The Mediating Role of Maternal Thyroid Hormones in the Relationship Between PAH Exposure and Neonatal Birth Weight

Previous studies have shown that exposure to polycyclic aromatic hydrocarbons (PAHs) is associated with decreased birth weight (BBW) in newborns and affects maternal thyroid hormone levels. However, the specific role of maternal thyroid hormones in the relationship between PAH exposure and decreased birth weight remains unclear. This study aimed to investigate the mediating role of maternal thyroid hormones in the relationship between maternal exposure to PAH and neonatal birth weight in 1335 pregnant women from rural Zunyi. The results showed a positive correlation between free triiodothyronine (FT3) and birth weight (beta = 41.90; 95% CI: 3.55, 80.25). Additionally, there was a non-linear negative correlation between total thyroxine (TT4) and free thyroxine (FT4), and an overall nonlinear positive correlation among thyroid-stimulating hormone (TSH), total triiodothyronine (TT3), TT4, FT3, FT4, and birth weight. Moreover, it was found that 1-OHPYR had a negative correlation with birth weight (beta = - 32.2; 95% CI: - 56.68, - 7.7). Furthermore, a U-shaped relationship was observed between PAH metabolites and birth weight. The study discovered that certain PAH metabolites, specifically 3-OHPH, 2-OHNAP, 2-OHPH, and 9-OHPH, had an impact on the secretion of TSH, TT3, TT4, FT3, and FT4. There was an inverted U-shaped relationship between PAH metabolites and TSH, TT3, TT4, and FT4, and a nonlinear negative relationship with FT3. Furthermore, the results of the structural equation modeling suggest that neonatal birth weight may be influenced by 1-OHNAP, 9-OHFLU, 4-OHPH, and 2-OHFLU via the FT3, TT4, and FT4 pathways. Therefore, it can be concluded that exposure to PAH metabolites has an impact on maternal pregnancy hormones, leading to a decrease in birth weight. It is worth noting that maternal pregnancy hormones play a partial role in mediating this effect. Therefore, it is crucial to reduce exposure to PAH sources and regulate maternal thyroid hormone levels.Graphical AbstractIn this study, we found that exposure to PAHs during pregnancy leads to lower birth weight, with maternal thyroid hormones playing a mediating role.