Copyright © 2014 by the Endocrine Society. Context: Maternal hypothyroidism in early pregnancy is associated with adverse outcomes, but not consistently across studies. First trimester screening for chromosomal anomalies is routine in many centers and provides an opportunity to test thyroid function. Objective: To determine if thyroid function tests performed with first trimester screening predicts adverse pregnancy outcomes. Design, Participants and Setting: A cohort study of 2411 women in Western Australia with singleton pregnancies attending first trimester screening between 9 and 14 weeks gestation. Outcome Measures: We evaluated the association between TSH, free T4, free T3, thyroid antibodies, free beta human chorionic gonadotrophin (β-hCG) and pregnancy associated plasma protein A (PAPP-A) with a composite of adverse pregnancy events as the primary outcome. Secondary outcomes included placenta previa, placental abruption, pre-eclampsia, pregnancy loss after 20 weeks gestation, threatened preterm labor, preterm birth, small size for gestational age, neonatal death, and birth defects. Results: TSH exceeded the 97.5th percentile for the first trimester (2.15 mU/L) in 133 (5.5%)women, including 22 (1%) with TSH above the nonpregnant reference range (4 mU/L) and 5 (0.2%) above 10 mU/L. Adverse outcomes occurred in 327 women (15%). TSH and free T4 did not differ significantly between women with or without adverse pregnancy events. On the multivariate analysis, neither maternal TSH >2.15 mU/L nor TSH as a continuous variable predicted primary or secondary outcomes. Conclusion: Testing maternal TSH as part of first trimester screening does not predict adverse pregnancy outcomes. This may be because in the community setting, mainly mild abnormalities in thyroid function are detected.