Ambient PM_2.5 and thyroid health: A cross-sectional study of 24,333 participants in Zhejiang, China

Evidence on the association between ambient fine particulate matter (PM_2.5) exposure and thyroid dysfunction remains inconsistent, with limited evidence from community-based populations encompassing both minors and adults while accounting for iodine nutrition. We aimed to evaluate the associations of ambient PM_2.5 exposure with complementary dimensions of thyroid health in a community-based population in Zhejiang Province, China. We conducted a cross-sectional study of 24,333 participants between 2022 and 2025. PM_2.5 exposure was estimated using participants' geocoded residential addresses. Linear and generalised linear mixed-effects models were used to estimate the associations of PM_2.5 with free thyroxine (FT4), thyroid-stimulating hormone (TSH), subclinical hypothyroidism, and autoimmune thyroiditis. Stratified analyses were conducted by age, sex, and body mass index (BMI). Our results showed that each 10 μg/m³ increase in PM_2.5 was associated with lower FT4 [-3.57% (95% confidence interval (CI): -4.39%, -2.75%)], higher TSH [12.15% (95% CI: 7.92%, 16.56%)], and higher odds of subclinical hypothyroidism [odds ratio (OR) 1.27 (95% CI: 1.06, 1.51)]. Exposure to PM_2.5 was not clearly associated with autoimmune thyroiditis. The association with lower FT4 was more pronounced among participants aged < 18 years and those with BMI < 18.5 kg/m², whereas the associations with higher TSH and higher odds of subclinical hypothyroidism were more pronounced among participants aged ≥ 50 years and those with BMI ≥ 24.0 kg/m². Overall, our findings suggest that higher ambient PM_2.5 exposure was associated with lower thyroid function.