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

BACKGROUND: 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. METHODS: 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. RESULTS: 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. CONCLUSIONS: 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.