Emerging evidence highlights the role of thyroid hormones in cancer, though findings are controversial. Research on thyroid-related traits in lung carcinogenesis is limited. Using UK Biobank data, we conducted bidirectional Mendelian randomization (MR) to assess causal links between lung cancer risk and thyroid dysfunction (hypothyroidism/hyperthyroidism) or function traits (free thyroxine [FT4], normal-range TSH). Furthermore, in the smoking-behavior stratified MR analysis, we evaluated the mediating effect of thyroid-related phenotypes on the association between smoking phenotype and lung cancer. We confirmed significant associations between lung cancer risk and hypothyroidism (hazards ratio [HR] = 1.14, 95% confidence interval [CI] = 1.03-1.26,