Invited commentary: Maternal plasma polybrominated diphenyl ethers and thyroid hormones--challenges and opportunities.

Thyroid hormones play a fundamental role in fetal and child development. While iodine deficiency-related maternal and child hypothyroidism may cause severe mental retardation, recent evidence suggests that milder forms of maternal hypothyroidism and hypothyroxinemia during pregnancy are also associated with altered neurodevelopment. On the other hand, hyperthyroidism during pregnancy has been associated with adverse fetal outcomes. Findings published by Abdelouahab et al. in the American Journal of Epidemiology (Am J Epidemiol. 2013;178(5):701-713) suggest that plasma concentrations of maternal polybrominated diphenyl ethers (PBDEs), which were used as flame retardants until recently and are detected in the tissues of virtually every North American, are associated with umbilical cord and maternal thyroid hormone levels during pregnancy. Although PBDEs have been consistently shown to reduce levels of free and total thyroxine in experimental animal studies, the direction of associations in human studies has been inconsistent. In this commentary, I discuss challenges beyond the factors often cited in the epidemiologic literature to explain inconsistent findings which more specifically apply to the study of PBDEs and thyroid hormones. These include the determination of iodine intake status, the method used to adjust for blood lipid concentrations, the measurement of free thyroid hormone levels, the possible effect of PBDE metabolites, and the potential for reverse causality.