Objective:To investigate the relationship between thyroid autoimmunity and pregnancy outcome in patients with unexplained recurrent abortion.Methods:A retrospective cohort study of 354 patients with normal thyroid function with recurrent abortion of unknown cause admitted to Sun Yat-sen Memorial Hospital, Sun Yat-sen University from January 2015 to June 2022 was used to detect thyroid antibody and thyroid function levels during pregnancy or early pregnancy. They were divided into TAI group ( n=144) and non-TAI group ( n=210) according to whether thyroid autoimmunity (TAI) was complicated or not. Tracking pregnancy outcomes. Results:Compared with the non-TAI group, the TAI group had a higher proportion of pregnancy outcomes resulting in miscarriage [42.4%(61/144) vs 27.1%(57/210), P=0.004]. In patients with unexplained recurrent abortion, TAI significantly increased the risk of spontaneous abortion [ OR(95% CI): 2.13(1.34, 3.41), P=0.001]. Positive TPOAb or TgAb also increased the risk of spontaneous abortion [ OR(95% CI): 2.18(1.37, 3.50), P=0.001; OR(95% CI): 2.33(1.31, 4.13), P=0.004]. TAI, TPOAb and TgAb had no significant interaction with age ( P=0.482, 0.724, 0.740). Conclusions:TAI is positively associated with the risk of spontaneous abortion in patients with unexplained recurrent abortion. TAI may be a potential risk factor for unexplained recurrent abortion, expanding the diagnosis and treatment of unexplained recurrent abortion.