Pesticide exposure and incident thyroid cancer among male pesticide applicators in agricultural health study

Background: Many pesticides are known to have thyroid-disrupting properties. However, few studies have evaluated the association between specific pesticide ingredients and risk of thyroid cancer. We investigated self reported pesticide use and incident thyroid cancer in the Agricultural Health Study (AHS), a large cohort of occupationally-exposed male pesticide applicators. Methods: The AHS is a prospective cohort of licensed pesticide applicators in Iowa and North Carolina. At enrollment (1993-1997) and follow-up (1999-2005), participants reported use of 50 pesticides. We characterized exposure as ever use (44 pesticides with >5 exposed cases) and by cumulative intensity-weighted lifetime days (22 pesticides with >10 exposed cases), a metric that accounts for factors that influence exposure. We estimated hazard ratios (HR) and 95% confidence intervals (CI) using Cox regression for incident thyroid (n = 85 cases) cancer among male participants using follow-up through 2014/2015. Results: Use of the fungicide metalaxyl (HR = 2.03, CI:1.16-3.52) and the organochlorine insecticide lindane (HR = 1.74, CI:1.06-2.84) was associated with increased risk of thyroid cancer. The herbicide chlorimuron-ethyl was inversely associated with risk when we restricted to papillary thyroid cancer, the most common subtype (HR = 0.52, CI:0.28-0.96). High use of the insecticide carbaryl (>median intensity-weighted days) was inversely associated with thyroid cancer (HR = 0.20, CI:0.08-0.53, p(trend) = 0.001). Conclusions: In this large cohort study, we observed increased risk of thyroid cancer associated with use of metalaxyl and lindane, and an inverse association with carbaryl. More work is needed to understand the potential role of these chemicals in thyroid carcinogenesis.