Exploring the causal relationship between long-term exposure to particulate matter 2.5 and risk of thyroid disorders: a Mendelian randomization analysis

Particulate matter 2.5 (PM_2.5) has been increasingly linked to thyroid dysfunction, yet causal evidence remains limited. This study employs a two-sample Mendelian Randomization (MR) approach to assess the causal relationship between long-term PM_2.5 exposure and thyroid disorders. Genetic instruments for PM_2.5 exposure were sourced from the UK Biobank, while outcome data on thyroid dysfunction and its subtypes were obtained from the FinnGen consortium. The primary method was inverse-variance weighted (IVW), supplemented by MR-Egger, weighted median, simple mode and weighted mode analyses. Pleiotropy and heterogeneity were evaluated with MR-Egger intercept, MR-PRESSO, and Cochran's Q test. The IVW method revealed a significant 77% increased risk of thyroid disorders per standard deviation rise in PM_2.5 exposure (OR = 1.77, 95% CI: 1.29-2.43, <i>p</i> = 3.93 × 10^-4). Notably, a strong causal association was identified with autoimmune hypothyroidism (OR = 1.83, 95% CI: 1.26-2.64, <i>p</i> = 1.33 × 10^-3), but not with autoimmune hyperthyroidism or other subtypes. Sensitivity analyses showed no pleiotropy or heterogeneity. These findings support a causal role of PM_2.5 in , thyroid dysfunction and highlight the importance of reducing air pollution to protect thyroid health.