The purpose of this study was to explore the impact of maternal thyroid hormone dysfunction in late pregnancy on birth outcomes in a Chinese population. We retrospectively examined hospitalisation records and laboratory data between April 2016 and March 2017 and obtained results from 11,564 consecutive pregnant women with singleton births in which serum thyroid hormone had been examined together with birth outcomes. We assessed the association between maternal thyroid level and dysfunction with adverse birth outcomes based on regression analysis. Hyperthyroidism was associated with an increased risk of preterm birth (PTB, adjusted OR: 2.41, 95% CI: 1.83–3.17) and hypothyroidism was associated with an increased risk of small for gestational age (SGA, adjusted OR: 1.56, 95% CI: 1.10–2.22), while hyperthyroxinaemia was associated with a decreased risk of large for gestational age (LGA, adjusted OR: 0.64, 95% CI: 0.45–0.90). In addition, compared to women with normal FT3 and TSH (≥the 5th and ≤the 95th percentiles), women with high free triiodothyronine (FT3 >the 95th percentile) and low thyroid-stimulating hormone (TSH <the 95th percentile) had a 4.02- fold higher risk of PTB (95% CI: 2.05–7.88), and women with low FT3 and high TSH had a 4.22- fold greater risk of SGA (95% CI: 1.59–11.23). Our study supports associations between multiple types of maternal thyroid dysfunction in late pregnancy and adverse birth outcomes.