Association Between Hashimoto's Thyroiditis and Rheumatoid Arthritis: A Bidirectional Mendelian Randomization Study.

Background: We aim to investigate the possible causal association between Hashimoto's thyroiditis (HT) and rheumatoid arthritis (RA) using Mendelian randomization (MR) methods. Methods: A bidirectional MR analysis was conducted to evaluate the causal association between HT and RA. We obtained summary statistics data from two extensive genome-wide association studies (GWAS) comprising 15,654 cases of HT and 14,361 cases of RA. The primary effect estimate utilized in this study was the inverse-variance weighted (IVW) method. To ensure the reliability and stability of the results, we employed several additional methods for testing, including MR-Egger, weighted median, simple mode, weighted mode, and MR-PRESSO. Results: Our study revealed compelling evidence of bidirectional causality between HT and RA. When HT was considered as an exposure factor and RA was considered as an outcome factor, this study revealed a positive correlation between HT and RA (IVW: odds ratio [OR] = 2.4546, 95% confidence interval [CI], 1.1473-5.2512; p = 0.0207). Conversely, when we examined RA as the exposure factor and HT as the outcome factor, we still found a positive correlation between them (IVW: OR = 1.2113, 95% CI, 1.1248-1.3044; p = 3.9478 × 10^-7). Conclusions: According to our research findings, there exists a bidirectional positive causal relationship between HT and RA among European populations. This implies that individuals with HT have an elevated risk of developing RA, and conversely, individuals with RA have an increased risk of developing HT.