Exposure to nitrogen dioxide and the risk of thyroid disorders: Two nationwide population-based studies in the UK and China.

BACKGROUND: Nitrogen dioxide (NO2), a major traffic-related air pollutant, is associated with adverse health effects; however, its relationship with thyroid disorders remains poorly characterized. OBJECTIVE: To investigate the associations between long-term NO2 exposure and thyroid-related outcomes in two large population-based studies from the UK and China. METHODS: In the UK Biobank, annual NO2 concentrations at baseline residential addresses were estimated using land-use regression models, and incident thyroid disorders were identified from ICD-10-codes in linked hospital inpatient records. In the Thyroid Disorders, Iodine Status and Diabetes Epidemiological Survey (TIDE), 5-year average NO2 concentrations were derived from gridded modeled data, and thyroid-related outcomes were assessed using biomarkers and ultrasonography. Cox proportional hazards models were applied to the UK Biobank data and weighted logistic regression models to TIDE data, adjusting for demographic, socioeconomic, lifestyle, and study-specific clinical factors. False discovery rate (FDR) correction was applied to primary continuous-exposure analyses. RESULTS: In fully adjusted models, each 10 μg/m3 increase in NO2 was associated with increased risks of hypothyroidism (hazard ratio [HR], 1.03; 95% confidence interval [CI], 1.00-1.06), goitre (HR, 1.12; 95% CI, 1.05-1.19), hyperthyroidism (HR, 1.07; 95% CI, 1.01-1.14), and thyroiditis (HR, 1.21; 95% CI, 1.05-1.40) in the UK Biobank. In TIDE, the same exposure increase was associated with higher odds of autoimmune thyroiditis (odds ratio [OR], 1.04; 95% CI, 1.01-1.06) and thyroid nodules (OR, 1.12; 95% CI, 1.09-1.15). All associations remained significant after FDR correction. CONCLUSIONS: Higher long-term NO2 exposure was associated with multiple thyroid-related outcomes across populations in the UK and China.