The association between urinary levels of organic phosphorus insecticide exposure and subclinical thyroid disorders

Abstract

Objective

Studies have shown that exposure to organophosphorus pesticides (OPPs) may disrupt thyroid endocrine function in animal models and in agroforestry practitioners, leading to subclinical hyperthyroidism (SHyper). However, the relationship between exposure to OPPs and SHyper in the general population remains unclear. This research aims to investigate the relationship between OPPs exposure and SHyper in the general population.

Methods

This was a retrospective cross-sectional study based on data collected from three cycles of the National Health and Nutrition Examination Survey (NHANES, 2001–2010), which ultimately analyzed 3425 participants who met the inclusion criteria. The study period is particularly relevant because it reflects the era of widespread chlorpyrifos (CPF) use in the United States before its subsequent ban, thereby providing important historical context for understanding OPP exposure levels. OPPs exposure was estimated by measuring urinary composition of two OPPs metabolites (3,5,6-trichloropyridinol and paranitrophenol). Logistic regression models were employed to assess the correlation between OPPs metabolites and SHyper. Subgroup analyses were conducted based on gender, age, body mass index (BMI), and household income, and interactions with OPPs were investigated. The two metabolites were categorized into quartiles, and trend analysis was performed. Restrictive cubic spline models were used to fit the relationship between the two metabolites and the risk of SHyper. Sensitivity analyses were conducted by excluding participants who consumed alcohol and those with iodine deficiency to validate the model's stability.

Results

Adjusted logistic regression analyses revealed significant positive associations between both para-nitrophenol (PNP) and 3,5,6-trichloropyridinol (TCPy) with subclinical hyperthyroidism (SHyper). Participants in the second, third, and highest quartiles of PNP exposure exhibited a progressively increased risk of SHyper compared to those in the lowest quartile.Stratified analyses demonstrated that TCPy’s association with SHyper was particularly pronounced in specific subgroups: individuals aged 12–64 years, males, those with household incomes < $20,000/year, and participants with obesity (BMI ≥30 kg/m²). Similarly, PNP was consistently associated with elevated SHyper risk across all stratified populations. Sensitivity analyses confirmed the robustness of these associations, as the relationships between PNP/TCPy and SHyper persisted even after excluding alcohol consumers or iodine-deficient individuals.

Conclusions

Our study suggests that exposure to OPPs is associated with an increased risk of SHyper, an association reflected by the observed correlations between urinary metabolites (PNP and TCPy) and SHyper.