Associations of cord serum polybrominated diphenyl ether (PBDE) mixture with birth outcomes and mediating role of thyroid function: Evidence from the Sheyang Mini birth cohort study

Background Polybrominated diphenyl ethers (PBDEs), a series of worldwide applied flame retardants, may influence fetal growth and interfere with thyroid function. The study intended to explore the relationship between in-utero exposure to PBDE mixture and newborn anthropometric indexes and to further examine the potential mediating role of thyroid function. Methods Demographics and laboratory measures of 924 mother-infant pairs were obtained from the database of the Sheyang Mini Birth Cohort Study. We applied gas chromatography-mass spectrometry (GC-MS) and electrochemiluminescence immunoassay to measure nine PBDE congeners and seven thyroid function parameters in umbilical cord serum samples, respectively. We fitted generalized linear models and Bayesian kernel machine regression (BKMR) to evaluate associations of lipid-adjusted cord serum PBDEs, as individuals and as a mixture, with newborn anthropometric and cord serum thyroid function parameters. We applied causal mediation analysis to test our hypothesis that thyroid function parameters act as a mediator between PBDEs and birth outcomes. Results The molarity of cord serum ∑9PBDE had a median value of 31.23 nmol/g lipid (IQR 19.14 nmol/g lipid, 54.77 nmol/g lipid). BDE-209 was the most dominant congener. Birth length was positively associated with both single exposure to BDE-28 and cumulative exposure to PBDEs. Correspondingly, ponderal index (PI) was negatively associated with BDE-28 and the total effects of PBDE mixture. Free triiodothyronine had a negative trend with BDE-209 and PBDE mixture. In the sex-stratified analysis, BDE-153 concentrations were positively correlated with PI among males (β = 0.03; 95%CI: 0.01, 0.05; P = 0.01) but not among females. Cord serum thyrotropin mediated 14.92% of the estimated effect of BDE-153 on PI. Conclusions In-utero mixture exposure to PBDEs was associated with birth outcomes and thyroid function. Thyroid function might act as a mediator in the process in which PBDEs impact the growth of the fetus.