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BACKGROUND: Though evidence suggests that some pesticides may have thyroid-disrupting properties, prospective studies of associations between specific pesticides and incident thyroid disease arc limited.

OBJECTIVE: We evaluated associations between use of specific pesticides and incident hypothyroidism in private pesticide applicators in. the Agricultural Health Study (AHS).

METHODS: Self -reported incident hypothyroidism (n = 829 cases) was studied in relation to ever-use and intensity- weighted cumulative days of pesticide use at study enrollment. We estimated adjusted hazard ratios (HRs) and 95% confidences intervals (Cl) using Cox proportional hazards models applied to 35,150 male and female applicators followed over 20 y. All models were stratified by suite and education to meet proportional hazards assumptions (p <= 0.10 for age x covariate interactions). Models of pesticides that did not meet proportional hazards assumptions were stratified by median attained age (62 y).

RESULTS: Hypothyroidism risk was significantly increased with ever- vs. never -use of four organochlorine insecticides (aldrin, heptachlor, and lindane among participants with attained age >62 y; chlordane in all participants), four organophosphate insecticides (coumaphos in those >62 y; diazinon, dichlorvos, and malathion in all participants) and three herbicides (dicamba, glyphosate, and 2,4-D M all participants). HRs ranged from 1.21; 9.57,3 CI: 1.04, 1.41 (chlordane) to 1.54; 95% CI: 1.23, 19.4 (lindane in those >62 y). Hypothyroidism risk was greatest among those with higher intensity weighted lifetime days of using chlordane, lindane, coumaphos (over age 62), diazinon, permethrin, and 2,4-D.

CONCLUSIONS: Our findings support associations between exposure to several pesticides and increased hypothyroidism risk. These findings are generally consistent with prior analyses of prevalent hypothyroidism in the AHS.