Objectives:　Thyroid　dysfunction　is　commonly　associated　with　the　risk　of　infertility　in　both　females　and　males.　However,　recent　randomized　controlled　trials　have　demonstrated　that　thyroid　function　levels　in　females　are　not　significantly　related　to　infertility,　and　evidence　on　the　association　between　male　thyroid　function　and　infertility　is　limited.　We　aim　to　investigate　the　association　between　thyroid　function　levels　and　infertility　in　both　females　and　males.　Method:　A　two-sample　Mendelian　randomization　study　was　conducted　using　four　methods,　with　the　inverse　variance　weighted　method　(IVW)　as　the　primary　approach.　Data　on　thyroid　function　as　the　exposure　were　obtained　from　the　ThyroidOmics　Consortium　and　UK　Biobank,　including　over　700,000　individuals　from　a　large　meta-analysis　of　genome-wide　association　studies　for　thyroid　function　and　dysfunction.　The　outcome　data　for　infertility　in　both　sex　encompassed　more　than　70,000　individuals　from　the　FinnGen　Consortium.　All　participants　were　adults　of　European　ancestry.　The　MR　Egger　regression　intercept　and　Cochran’s　Q　test　were　employed　to　evaluate　directional　pleiotropy　and　heterogeneity.　Results:　The　results　indicated　no　causal　effect　of　thyroid-stimulating　hormone　(TSH)　and　free　tetraiodothyronine　(fT4)　on　female　and　male　infertility.　Furthermore,　no　causal　association　between　hypo-　and　hyperthyroidism　and　infertility　were　identified.　Notably,　we　observed　a　causal　relationship　between　high　TSH　and　endometriosis-related　infertility　(OR=0.82,　95%　CI:　0.74–0.91,　P　=　1.49E-04).　Conclusions:　This　study　did　not　find　evidence　for　casual　relationship　between　thyroid　function　levels　and　risk　of　infertility.　The　findings　suggest　that　overall　thyroid　function　levels　may　not　be　a　significant　predictor　of　infertility　risk.　Copyright　©　2024　Liu,　Qiu,　Jiang,　Long,　Zhu,　Chen　and　Wen.