BACKGROUND

Organophosphates are frequently applied insecticides that inhibit acetylcholinesterase (AChE) activity resulting in cholinergic overstimulation. Limited evidence suggests that organophosphates may alter thyroid hormone levels, although studies have yielded inconsistent findings. We aimed to test the associations between AChE activity, a physiological marker of organophosphate exposure, and thyroid function in adolescents.

METHODS

We included information of 80 adolescent participants (ages 12-17y in 2016, 53% male) growing up in agricultural settings in Ecuador. We measured fingerstick erythrocytic AChE activity and hemoglobin concentration, and concurrent serum thyroid stimulating hormone (TSH) and free-T4 (fT4) concentrations. General linear models were used to test associations which adjusted for demographic and anthropometric variables. TSH associations were further adjusted for fT4.

RESULTS

The mean (SD) AChE, TSH and fT4 levels were 3.77 U/mL (0.55), 2.82 μIU/ml (1.49) and 1.11 ng/dl (0.13), respectively. Lower AChE activity, indicating greater organophosphate exposure, was marginally associated with greater fT4 concentrations (difference per SD decrease in AChE activity (β) = 0.03 ng/dL, [90% CI: 0.00, 0.06]) but not with TSH (β = -0.01 μIU/ml, [-0.38, 0.36]). Gender modified the AChE-TSH association (p = 0.03). In girls, lower AChE activity was associated with higher fT4 levels (β=0.05 ng/dL [0.01, 0.10]) and lower TSH concentrations (β = -0.51 μIU/ml, [-1.00, -0.023]). No associations were observed in boys.

DISCUSSION

These cross-sectional findings suggest that alterations in the cholinergic system from organophosphate exposures can increase fT4 levels coupled with a beyond-compensatory downregulation of TSH in female adolescents. This is the first study to characterize these associations in adolescents.