Bidirectional Mendelian randomization analysis of the causal relationship between inflammatory bowel disease and Hashimoto thyroiditis

This study aimed to explore the bidirectional causal relationship between inflammatory bowel disease (IBD) and Hashimoto thyroiditis (HT) using Mendelian randomization (MR) analysis. While both conditions are characterized by persistent inflammation and immune dysregulation, the direction of causality remains unclear. We performed a 2-sample bidirectional MR analysis using summary statistics from genome-wide association studies for IBD and HT. Genetic variants strongly associated with IBD and HT were selected as instrumental variables for forward and reverse MR analyses, respectively. Various MR methods, including inverse-variance weighted (IVW), MR-Egger regression, weighted median, and weighted mode, were employed to assess causal relationships. Sensitivity analyses were conducted to evaluate the reliability of results, including tests for pleiotropy and heterogeneity. Data were sourced from individuals of European descent to minimize population stratification bias. In the forward MR analysis, no strong evidence of a causal effect of IBD on HT was found, with the IVW method yielding an odds ratio (OR) of 0.9617 (95% CI: 0.7549–1.2251; P = .7519) Similarly, in the reverse MR analysis, no significant causal effect of HT on IBD was observed, with the IVW method showing an OR of 0.9991 (95% CI: 0.9693–1.0298, P = .9527). Sensitivity analyses confirmed the absence of heterogeneity or pleiotropic effects that could influence the results. The MR-PRESSO analysis did not detect any outlier SNPs. This bidirectional MR analysis provides no evidence for a causal relationship between IBD and HT in either direction.