Pesticide exposure and thyroid hormones during pregnancy in the Infants' Environmental Health Study (ISA)

Introduction: Growing evidence suggests that pesticide exposure may disrupt thyroid hormones, but data in humans are limited. We studied the association of pesticide exposure and thyroid hormones in pregnant women living near banana plantations with extensive pesticide use in Costa Rica. Methods: We collected repeated serum and urine samples from 395 pregnant women (n=631) from the ISA study, a birth cohort study. We measured free triiodothyronine (FT3), free thyroxine (FT4), and thyroid stimulating hormone (TSH) in serum. We also measured pesticide metabolites for chlorpyrifos (TCPy), mancozeb (ETU), pyrimethanil (OH-P), 2,4-D (2,4-D) and synthetic pyrethroids such as cypermethrin, deltamethrin, and cyfluthrin (3-PBA, DCCA) in urine. We examined the associations of log-transformed specific-gravity corrected pesticide metabolite concentrations with thyroid hormones individually using linear mixed-effect models. All models were adjusted for gestational age, maternal age and urinary density at blood and urine sample collection. Results: Median (25-75th percentile) TCPy, ETU, OH-P, 2,4-D, 3-PBA, and DCCA concentrations were 1.5 (0.8-2.6), 2.8 (1.8-4.5), 0.3 (0.1-0.8), 0.3 (0.1-0.5), 0.6 (0.3-1.4), and 1.0 (0.4-2.2) μg/L, respectively. Median (25-75th percentile) FT3, FT4 and TSH concentrations were 4,6 (4,2-4,9), 14 (12.7-15.1) pmol/L and 1.5 (1.1-2.0) mIU/L, respectively. Higher TCPy concentrations were associated with higher FT3 [β per 10-fold increase in exposure = 0.2 pmol/L (95% CI: 0.05, 0.3)], whereas higher 3-PBA concentrations were associated with higher FT4 [0.3 pmol/L (0.02, 0.7)]. Higher OH-P concentrations were associated with lower TSH [-0.1 mIU/L (-0,2, -0.01)]. In contrast, ETU and 2,4-D concentrations were not associated with thyroid hormones. Conclusion: Our findings suggest that chlorpyrifos and synthetic pyrethroid pesticides may alter thyroid function and are consistent with results from previous epidemiological and animal studies.