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Thyroid hormones are essential for proper neurodevelopment in early life. There is evidence that exposure to polybrominated diphenyl ethers (PBDEs) affects thyroid function, but previous studies have been inconsistent, and no studies among children have been conducted in the United States where PBDE levels are particularly high. Serum levels of seven PBDE congeners and thyroid hormones and other thyroid parameters were measured in 80 children aged 1-5 years from the southeastern United States between 2011 and 2012. Parents of the children completed questionnaires with details on demographics and behaviors. Multivariate linear regression models were used to estimate the associations between serum PBDE levels, expressed as quartiles and as log-transformed continuous variables, and markers of thyroid function. BDE-47, 99, 100 and 153 were detected in > 60% of samples, and were summed (Sigma PBDE). PBDE congeners and Sigma PBDE were positively associated with thyroid-stimulating hormone (TSH). A log-unit increase in IPBDE was associated with a 22.1% increase in TSH (95% CI: 2.0%, 47.7%). Compared with children in the lowest quartile of Sigma PBDE exposure, children in higher quartiles had greater TSH concentrations as modeled on the log-scale (second quartile: (beta=0.32, 95% confidence interval (CD: 0.09, 0.74; third quartile: beta=0.44, 95% CI: 0.04, 0.85; and fourth quartile: (beta=0.49, 95% CI: 0.09, 0.89). There was also a tendency toward lower total T-4 and higher free T-3 with increasing PBDE exposure. Results suggest that exposure to PBDEs during childhood subclinically disrupts thyroid hormone function, with impacts in the direction of hypothyroidism. (c) 2016 Elsevier Inc. All rights reserved.