No Genetic Causal Association between Periodontitis and Autoimmune Thyroiditis: A Bidirectional Two-Sample Mendelian Randomization Analysis

Background Recent studies have reported associations between periodontitis and autoimmune disorders; however, it remains unclear whether the association between periodontitis and autoimmune thyroiditis (AIT) is causal. The purpose of this study was to investigate the causal association between periodontitis and AIT. Methods We conducted a two-sample bidirectional Mendelian randomization (MR) analysis using publicly available genome-wide association study (GWAS) statistics. The single-nucleotide polymorphisms (SNPs) for AIT were derived from 244 patients and 187,684 controls. The pooled periodontitis association data included up to 17,353 individuals. The inverse-variance weighted (IVW) method was used for the primary analysis. We applied two complementary methods, the weighted median and MR-Egger. Then, these causal estimates were expressed as dominance ratios [odds ratios (ORs)]. Results Genetically determined AIT did not have a causal effect on probing depth (PD) (IVW OR = 0.998, 95% confidence interval (CI) = 0.948 ~ 1.051, p = 0.952). Furthermore, we did not find a significant causal effect of PD on AIT in the reverse MR analysis (IVW OR = 1.044, 95% confidence interval (CI) = 0.789 ~ 1.382, p = 0.759). The MR-Egger regression and weighted median results were consistent with those of the IVW method. Horizontal pleiotropy was unlikely to distort the causal estimates according to the sensitivity analysis. Conclusion Within the limits of MR studies, the outcomes neither supported periodontitis affecting autoimmune thyroiditis nor autoimmune thyroiditis affecting periodontitis.