AbstractObjectiveHypothyroidism has been associated with pregnancy complications, but uncertainty prevail regarding the severity and the role of thyroid autoimmunity. This study aimed to evaluate adverse pregnancy outcomes by exposure to maternal hypothyroidism and thyroid autoimmunity.DesignRetrospective cohort study.Patients14,744 singleton pregnancies from the North Denmark Region Pregnancy Cohort (2011−2015).MeasurementsMaternal thyroid stimulating hormone (TSH), thyroid peroxidase antibodies (TPO‐Ab), and thyroglobulin antibodies (Tg‐Ab) were retrospectively measured in early pregnancy blood samples (ADVIA Centaur XPT, Siemens Healthineers). Adjusted odds ratio (aOR) with 95% confidence interval (CI) was used to estimate associations between maternal hypothyroidism (TSH cut‐offs: 6.0 and 10 mIU/L), thyroid autoimmunity (TPO‐Ab cut‐off: 60 U/ml, Tg‐Ab cut‐off: 33 U/ml), and adverse pregnancy outcomes.ResultsPregnancy outcomes were 93.2% live births, 6.5% spontaneous abortions, and 0.3% stillbirths. The frequency of spontaneous abortion was 6.5% when TSH was below 6.0 mIU/L, 6.5% when above 6.0 mIU/L (aOR 1.0 [95% CI: 0.5−2.0]), and 12.5% when above 10 mIU/L (aOR: 2.0 [95% CI: 0.8−5.2]). For outcome of preterm birth, the frequency was 5.4% when TSH was below 6.0 mIU/L, 7.8% when above 6.0 mIU/L (aOR 1.5 [95% CI: 0.7−2.9]), and 11.4% when above 10 mIU/L (aOR: 2.6 [95% CI: 0.9–7.3]). No association was found between thyroid autoantibodies and spontaneous abortion (TPO‐Ab: aOR: 1.0 [0.8−1.3], Tg‐Ab: 1.0 [0.8−1.2]) or preterm birth (TPO‐Ab: aOR: 1.0 [0.8–1.2], Tg‐Ab: 0.9 [0.7–1.2]).ConclusionA high frequency of adverse pregnancy outcomes was seen among pregnancies exposed to maternal TSH above 10 mIU/L, whereas no association with thyroid autoantibodies was seen.