Thyroid hormone levels associate with exposure to polychlorinated biphenyls and polybrominated biphenyls in adults...

Michigan residents were directly exposed to endocrine-disrupting compounds, polybrominated biphenyl (PBB) and polychlorinated biphenyl (PCB). A growing body of evidence suggests that exposure to certain endocrine-disrupting compounds may affect thyroid function, especially in people exposed as children, but there are conflicting observations. In this study, we extend previous work by examining age of exposure's effect on the relationship between PBB exposure and thyroid function in a large group of individuals exposed to PBB.

Linear regression models were used to test the association between serum measures of thyroid function (total thyroxine (T₄), total triiodothyronine (T₃), free T₄, free T₃, thyroid stimulating hormone (TSH), and free T₃: free T₄ ratio) and serum PBB and PCB levels in a cross-sectional analysis of 715 participants in the Michigan PBB Registry.

Higher PBB levels were associated with many thyroid hormones measures, including higher free T₃ (p = 0.002), lower free T₄ (p = 0.01), and higher free T₃: free T₄ ratio (p = 0.0001). Higher PCB levels were associated with higher free T₄ (p = 0.0002), and higher free T₃: free T₄ ratio (p = 0.002). Importantly, the association between PBB and thyroid hormones was dependent on age at exposure. Among people exposed before age 16 (N = 446), higher PBB exposure was associated with higher total T₃ (p = 0.01) and free T₃ (p = 0.0003), lower free T₄ (p = 0.04), and higher free T₃: free T₄ ratio (p = 0.0001). No significant associations were found among participants who were exposed after age 16. No significant associations were found between TSH and PBB or PCB in any of the analyses conducted.

This suggests that both PBB and PCB are associated with thyroid function, particularly among those who were exposed as children or prenatally.