ABSTRACT Background Whether hypothyroidism and cardiovascular disease (CVD) are causally related remains ambiguous. A bidirectional and multivariable two‐sample Mendelian Randomization (MR) approach was employed to elucidate these associations. Methods All genome‐wide association study (GWAS) data were obtained from the IEU Open GWAS project. For the MR estimates, inverse variance weighted (IVW), MR‐Egger regression, weighted median, simple mode, and weighted mode were utilized to assess the causal relationships. Multivariate MR (MVMR) analyses were conducted to explore the direct effects. A series of sensitivity analyses was used to ensure the robustness of our findings. All analyses were replicated in the validation samples. Results Higher genetically predicted hypothyroidism was causally associated with increased the risk of coronary artery disease (CAD, OR = 3.220, 95% CI = 1.082–9.583, P = 0.036), myocardial infarction (MI, OR = 3.430, 95% CI = 1.088–10.819, P = 0.035) and peripheral atherosclerosis (PAS, OR = 31.852, 95% CI = 5.987–169.457, P < 0.001). Reversely, CAD (OR = 1.005, 95% CI = 1.001–1.009, P = 0.02) and MI (OR = 1.006, 95% CI = 1.002–1.011, P = 0.004) may be more likely to promote the onset of hypothyroidism. Moreover, no significant causal effects of hypothyroidism on heart failure (HF) and small vessel stroke (SVS) were observed in the discovery samples. However, in the validation samples, hypothyroidism was found to be causally associated with HF (OR = 1.429, 95% CI = 1.240–1.647, P < 0.001) and SVS (OR = 7.297, 95% CI = 1.440–36.984, P = 0.016). For the reverse MR from PAS, HF, and SVS to hypothyroidism, no significant causal associations were identified. Conclusions This study reveals suggestive evidence of the bidirectional causal associations between hypothyroidism and CAD and MI. Moreover, hypothyroidism may increase the risk of PAS.
Bai et al. (2026) studied this question.