Objective: To determine the association of thyroglobulin antibodies (TG-Ab) and thyroid peroxidase antibodies (TPO-Ab) with recurrent spontaneous abortion in a euthyroid, nonpregnant population of women in Iran. Methods: In this case-control study conducted between November 2003 and September 2006 in Tehran, Iran, nonpregnant women with a history of 3 or more consecutive pregnancy losses and age-matched, healthy parous women without a history of reproductive problems were assessed. Thyroid function tests were performed, which included assessment of thyroid-stimulating hormone, triiodothyronine, thyroxine, and the presence of TG-Ab and TPO-Ab. Results: A total of 641 patients and 269 controls were included. Mean age (± SD) was 30.6 ± 6.4 years (range, 16-51 years) in the patient group and 30.05 ± 6.6 years (range, 18-48 years) in the control group. Thyroid antibodies were present in 157 of 641 patients (24.5%) and in 34 of 269 controls (12.6%) (P<.001). The presence of thyroid antibodies was significantly associated with recurrent abortion independent of the impact of age with an odds ratio of 2.24 (95% confidence interval, 1.5-3.35). Conclusions: In this population of women in Iran, TG-Ab and TPO-Ab were identified more frequently in women with recurrent abortions compared with controls, and thyroid autoimmunity was independently associated with a higher risk of recurrent abortion.