Elevated PCB levels in cord blood are associated with lower thyroid hormones in babies.
Herbstman JB, A Sjödin, BJ Apelberg, FR Witter, RU Halden, DG Patterson Jr, SR Panny, LL Needham and LR Goldman. 2008. Birth delivery mode modifies the associations between prenatal PCB and PBDE and neonatal thyroid hormone levels. Environmental Health Perspectives 116:1376–1382.
In a first-of-its-kind study, higher levels of polychlorinated biphenyls (PCB) and polybrominated diphenylethers (PBDE) in umbilical cord blood was associated with lower thyroid hormone levels in children born naturally (no induction or emergency C-section).
Lower levels of thyroid hormones are associated with delays in brain development. Finding an association between PCBs and PBDEs and thyroid hormone levels at birth is important because exposure during fetal development could make the individual more sensitive to the long-term health concerns of these compounds than the general population.
A critical part of this study was comparing the method of childbirth to thyroid hormone levels. Thyroid hormones fluctuate wildly depending on how the baby is delivered. Levels usually increase in the more than 70% of births that require medical intervention. Previous studies that did not account for the method of delivery may have underestimated, or even missed, the association between PCB and PBDE levels and reduced thyroid hormone levels.
In the babies in this study, elevated PCB and PBDE levels were associated with lower thyroid hormones, particularly thyroxine or T4. Thyroid hormones are highly regulated in the womb, so they occur at just the right levels for proper development. PCBs and some PBDE flame retardants are banned, but these ubiquitous and persistent compounds remain in the environment and are chemically similar to the thyroid hormone T4.