BACKGROUND:Several observational studies have found a close relationship between thyroid function levels and sarcopenia,but the causal relationship between thyroid function levels and the onset of sarcopenia is not yet clear. OBJECTIVE:To investigate the causal relationship between thyroid function levels and sarcopenia using a two sample Mendelian randomization method. METHODS:A two sample Mendelian randomization analysis was conducted using genome-wide association study data on thyrotropin,free triiodothyronine,free tetraiodothyronine,subclinical hyperthyroidism,subclinical hypothyroidism,and four related phenotypes of sarcopenia-lefthand grip strength,right hand grip strength,limb lean mass,and gait speed.The inverse-variance weighted method,weighted median method,simple mode method,weighted median estimator method,and MR Egger regression method were used as analysis methods,while heterogeneity test,pleiotropy test,MR-PRESSO,leave-one-out method,funnel plot and other methods were used for sensitivity analysis. RESULTS AND CONCLUSION:Elevated levels of thyroid-stimulating hormone increased left-(β=0.02,SE=0.01,P=0.01)and right-handed grip strength(β=0.02,SE=0.01,P=0.01),an increase in free triiodothyronine decreased left-(β=-0.06,SE=0.02,P=9.5×10-5)and right-handed grip strength(β=-0.07,SE=0.02,P=9.3×10-5),and subclinical hyperthyroidism decreased gait speed(β=-4.4×10-3,SE=1.7×10-3,P=0.01).The sensitivity analysis results were basically consistent with the main analysis results.To conclude,an increase in thyroid-stimulating hormone is a protective factor for sarcopenia,and elevation of free triiodothyronine and subclinical hyperthyroidism may increase the risk of sarcopenia.