Birth delivery mode modifies the associations between prenatal polychlorinated biphenyl (PCB) and polybrominated diphenyl ether (PBDE) and neonatal thyroid hormone levels

Background: Developing infants may be especially sensitive to hormone disruption from chemicals including polychlorinated biphenyls (PCBs) and polybrominated diphenyl ethers (PBDEs). Objective: We investigated relationships between cord serum levels of PCBs and PBDEs and thyroid hormones measured in cord blood serum and neonatal blood spots. Methods: We measured PCBs and PBDEs, thyrotropin (TSH), thyroxine (T₄) and free T₄ (FT₄) in cord blood serum from 297 infants who were delivered at the Johns Hopkins Hospital in 2004-2005. We abstracted results of total T₄ (TT₄) measured in blood spots collected in the hospital and at neonatal visits. We used delivery mode (augmented vaginal deliveries and nonelective cesarean deliveries) as a surrogate for intrapartum stress, which is known to alter cord blood thyroid hormones. Results: In the full study population, no compounds were associated with a change in average TSH, FT₄, or TT₄. BDE-100 was associated with increased odds of low cord TT₄, BDE-153 with increased odds of low cord TT₄ and FT₄, and no compounds were associated with increased odds of high TSH. For infants born by spontaneous, vaginal, unassisted deliveries, PCBs were associated with lower cord TT₄ and FT₄ and lower TT₄ measured in neonatal blood spots. PBDEs showed consistent but mainly nonsignificant negative associations with TT₄ and FT₄ measurements. Conclusions: Prenatal PCB and PBDE exposures were associated with reduced TT₄ and FT₄ levels among infants born by spontaneous, unassisted vaginal delivery. Intrapartum stress associated with delivery mode may mask hormonal effects of PCBs and PBDEs.