Objective To investigate the bidirectional causal relationship between endometriosis and thyroid cancer using bidirectional two-sample Mendelian randomization(MR).Methods Ge-nome-wide association study(GWAS)data for endometriosis were obtained from the FinnGen R11 database,and GWAS data for thyroid cancer were sourced from the European Bioinformatics Institute(EMBL-EBI).Single nucleotide polymorphisms(SNPs)that met the requirements were selected as instrumental variables.The inverse variance weighted(IVW)method was employed as the primary analytical approach,supplemented by MR-Egger regression,weighted median method,simple model,and weighted model results.Cochran's Q test,MR-Egger regression intercept analysis,and the leave-one-out method were used to assess the robustness of the results.Results Ultimately,25 SNPs high-ly associated with endometriosis and 3 SNPs highly associated with thyroid cancer were included in this study.The IVW analysis results of the forward MR analysis indicated no significant causal rela-tionship between endometriosis and the risk of thyroid cancer(P=0.50,OR=1.06,95％CI:0.88 to 1.29).This result was also supported by MR-Egger regression,simple model,weighted model,and weighted median analyses(P＞0.05).The IVW analysis results of the reverse MR analysis similarly showed no significant causal relationship between thyroid cancer and the risk of endometrio-sis(P=0.09,OR=1.03,95％CI,0.99 to 1.06).The findings from MR-Egger regression,sim-ple model,weighted model,and weighted median analyses all supported this conclusion(P＞0.05).Conclusion The MR analysis doesn't show a significant bidirectional causal relationship between endometriosis and thyroid cancer.