Exposure to a mixture of endocrine disrupting chemicals and thyroid function tests in pregnant women in the SELMA study

Based on experimental and human studies, endocrine disrupting chemicals (EDCs) can disrupt the thyroid hormone system. However, their association with thyroid function tests when considered as part of a chemical mixture is unknown.

We used data of 1970 pregnant women from the Swedish Environmental Longitudinal Mother and Child, Asthma and Allergy (SELMA) study to investigate the cross-sectional association between exposure to 26 chemical compounds with maternal thyroid function tests in early pregnancy, using Weighted Quantile Sum (WQS) regression.

Higher exposure to EDCs mixtures was associated with a lower FT3 [WQS Estimate per an IQR increase (95 % CI): -0.09 (-0.16 to -0.01), mostly driven by PCBs] and a lower TT3 [WQS Estimate per an IQR increase (95 % CI): -0.05 (-0.09 to -0.01), mostly driven by PFOS]. In addition, higher exposure to a mixture of short lived urinary based compounds was associated with a lower TT4/TT3 ratio while higher exposure to a mixture of persistent serum based compounds was associated with a higher TT4/TT3 ratio.

In this proof-of-principle analysis, we show that there could be an added benefit of analyzing thyroid hormone system disrupting EDCs using a mixture-based analysis approach. Our findings pave the way and provide hypotheses for future experimental and human studies to investigate the effects of EDCs as a mixture on the thyroid hormone system, revealing information on potential biological mechanisms explaining the associations from observational data.