Objective To investigate the bidirectional causal relationship between endometriosis and thyroid cancer using bidirectional two-sample Mendelian randomization (MR).

Methods Genome-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 highly 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 relationship 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 endometriosis (P=0.09, OR=1.03, 95%CI, 0.99 to 1.06). The findings from MR-Egger regression, simple 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.