Abstract: Objective To explore the causal relationship between thyroid diseases (hypothyroidism, hyperthyroidism) and preeclampsia using bidirectional two sample Mendelian randomization (MR) analysis. Methods Using summary data from genome-wide association studies (GWAS), MR analyses were conducted by the inverse-variance weighted (IVW) method, weighted median (WM), MR-Egger method, and maximum likelihood (ML) estimation to assess the causality between thyroid diseases and the risk of preeclampsia. Heterogeneity tests, sensitivity analyses, and pleiotropy assessments were also performed to ensure the robustness of the results. Results The MR analysis results showed that hypothyroidism (IVW: OR=3.804, 95% CI 1.121-12.911; P=0.032) and hyperthyroidism (IVW: OR=1.163, 95% CI 1.071-1.263; P<0.001) were causally associated with an increased risk of preeclampsia. However, in reverse MR analysis, no causal evidence was found between preeclampsia and increased risk of thyroid disease abnormalities. Additionally, no pleiotropy was detected in the instrumental variables, and leave-one-out analysis indicated the results were robust. Conclusions MR analysis revealed a causal relationship between hypothyroidism, hyperthyroidism and the increased risk of preeclampsia. Nevertheless, there is no causal relationship between preeclampsia and an increased risk of either thyroid disorder.