Association between urinary 3, 5, 6-trichloro-2-pyridinol, a metabolite of chlorpyrifos and chlorpyrifos-methyl, and serum T4 and TSH in NHANES 1999–2002

Thyroid hormones are vital to a host of human physiological functions in both children and adults. Exposures to chemicals, including chlorpyrifos, have been found to modify thyroid signaling at environmentally relevant levels in animal studies. The aim of this study was to examine circulating T4 and TSH levels in relation to urinary concentrations of 3, 5, 6-trichloro-2-pyridinol (TCPY), a metabolite of the organophosphorus insecticides chlorpyrifos and chlorpyrifos-methyl, using data from individuals 12 years and older from the U.S. National Health and Nutrition Examination Surveys (NHANES). NHANES datasets from 1999 to 2000 and 2001–2002 were combined, and individuals with thyroid disease, those taking thyroid medications, and pregnant women were excluded (N = 3249). Multivariable linear regression models for relationships between log-transformed urinary TCPY and serum total T4 or log (TSH) were constructed adjusting for important covariates. Models were stratified by sex and a categorical age variable (12–18, 18–40, 40–60, and > 60 years). In male participants, an interquartile range (IQR) increase in urinary TCPY was associated with statistically significant increases in serum T4 of 3.8% (95th CI 0.75 to 7.0) among those 12–18 years of age and 3.5% (95th CI 0.13 to 7.0) in the 18–40 year age group, relative to median T4 levels using unweighted models. An IQR increase in TCPY was also associated with decreases in TSH of 10.7% (− 18.7–2.05) among men 18–40 years old and 20.0% (95th CI − 28.9 to − 9.86) among men > 60 years old. Conversely, urinary TCPY was positively associated with TSH in females > 60 years of age. Further research to confirm these findings, elucidate mechanisms of action, and explore the clinical and public health significance of such alterations in thyroid hormones is needed.