Exploring the link between nitrate exposure and thyroid cancer: A nationwide state-level analysis

Context: Early-onset thyroid cancer incidence has been increasing, raising concerns about contributing factors. Objective: We aimed to investigate the role of nitrate contamination in drinking water as a contributor to early-onset thyroid cancer incidence. Methods: We designed an ecological study, analyzing population-based data from the National Program of Cancer Registries. We quantified the association between nitrate exposure and early-onset thyroid cancer with a set of Poisson generalized estimating equations (GEE). Analyses adjusted for current and 7-year lagged obesity rates and access to screening. Our nationwide study includes data from all 50 states in the United States. Patients between ages 0 to 14, 15 to 39, and 40 to 59 who were diagnosed with thyroid cancer between 2003 and 2022. States were categorized into 2 groups based on predicted groundwater nitrate levels. States with nitrate < 2.0 mg/L were classified as "Low," and states with levels ≥ 2.0 mg/L as "High." Population-adjusted cases of thyroid cancer incidence, with patients grouped by age and sex. Results: For the 0 to 14 or 40 to 59 age groups, we found no differences in thyroid cancer incidence by nitrate exposure. For ages 15 to 39, there were 41.6 (CI: 6.2, 77.1) more cases in high nitrate states, reflecting an 18.5% difference. Stratified by sex, in the 15 to 39 age group of females had 32.7 (CI: 6.3, 59.1) more cases and males had 8.2 (CI: 0.1, 16.4) more cases, reflecting a 17.8% and 20.4% difference respectively. Conclusion: Elevated exposure to groundwater nitrate may be a significant preventable contributor to thyroid cancer in adolescents and young adults.