Association of urinary pesticide metabolites and iodine with thyroid volume in school-aged children

Monitoring thyroid volume in children allows for non-invasive, intuitive, and rapid assessment of potential thyroid disorders, facilitating early detection in large-scale screenings without blood draws. In this study, we investigated associations between urinary pesticide metabolites (organophosphate, pyrethroid, and phenoxy carboxylic acid), urinary iodine, and thyroid volume in children aged 8-11 years across Guangzhou districts, China. We compared the spatial distribution of urinary pesticide metabolites among districts and assessed health risks using probabilistic Monte Carlo simulation. We evaluated associations between thyroid volume and multiple factors: urinary iodine, demographic variables, anthropometric measures, environmental factors, and urinary creatinine. The associations between pesticide metabolites and thyroid volume were assessed using multivariable linear regression, trend tests, and restricted cubic splines. The joint effects of multiple pesticide metabolites on thyroid volume were evaluated by the Bayesian kernel machine regression. The role of urinary iodine was explored through mediation and moderation analyses. The result revealed that 0.89 % of children had risk levels of parathion exposure. Thyroid volume showed positive correlations with age and body surface area but negative correlation with urinary iodine. Two pyrethroid metabolites, cis-3-(2,2-dichlorovinyl)-2,2-dimethylcyclopropane-1-carboxylic acid (cis-DCCA) and trans-3-(2,2-dichlorovinyl)-2,2-dimethylcyclopropane-1-carboxylic acid (trans-DCCA), demonstrated negative associations with thyroid volume after covariate adjustment. While thyroid volume decreased with higher pesticide metabolite concentrations, this joint effect was not statistically significant. The associations between thyroid volume and cis-DCCA and trans-DCCA were fully mediated by urinary iodine, but the metabolites' effects on thyroid volume remained consistent across different urinary iodine concentrations. The results indicated that pesticide exposure may influence thyroid volume through an iodine-mediated pathway and highlighted the importance of iodine status in thyroid health assessment.