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Overt thyroid dysfunction has been associated with adverse obstetric outcomes. However, less is known regarding subclinical hypothyroidism or thyroid autoimmunity and their relationship to pregnancy complications.The purpose of this study was to examine the association between prepregnancy anti–thyroid antibodies and subclinical hypothyroidism and preterm delivery, gestational diabetes mellitus, and preeclampsia.We conducted a secondary analysis of a prospective cohort of 18- to 40-year-old women with 1–2 previous pregnancy losses (n=1193) who participated in a multicenter randomized, placebo-controlled trial of low-dose aspirin. Prepregnancy levels of thyroid-stimulating hormone, free thyroxine, thyroglobulin antibody, and thyroid peroxidase antibody were measured. Relative risks and 95% confidence intervals were estimated with the use of generalized linear models with adjustment for age and body mass index.Among women with an ongoing pregnancy of >20 weeks estimated gestational age, there was no association between prepregnancy thyroid-stimulating hormone level (>2.5 vs ≤2.5 mIU/L) and preterm delivery (adjusted relative risk, 0.77; 95% confidence interval, 0.40–1.47), gestational diabetes mellitus (adjusted relative risk, 1.28; 95% confidence interval, 0.54–3.04), or preeclampsia (adjusted relative risk, 1.20; 95% confidence interval, 0.71–2.04). Similarly, among women with thyroid antibodies, there was no increase in the likelihood of preterm delivery (relative risk, 1.26; 95% confidence interval, 0.65–2.45), gestational diabetes mellitus (relative risk, 1.33; 95% confidence interval, 0.51–3.49), or preeclampsia (relative risk, 1.02; 95% confidence interval, 0.54–1.92), compared with women without these antibodies.Among women with 1–2 previous pregnancy losses, subclinical hypothyroidism and thyroid autoimmunity were not associated with an increased risk of preterm delivery, gestational diabetes mellitus, or preeclampsia. These data support current recommendations that low-risk asymptomatic women should not be screened routinely for thyroid dysfunction or autoimmunity.