Correlations Between Thyroid Hormone Levels of Women With Twin Pregnancies in the Third Trimester and Pregnancy Complications and Pregnancy Outcomes

Purpose: To explore changes in thyroid hormone levels of women with twin pregnancies in the third trimester under different pregnancy complications and pregnancy outcomes and to analyze their associated correlations. Methods: A total of 646 women with twin pregnancies who attended their first prenatal visit and received subsequent prenatal care in our hospital were enrolled in this study. Their thyroid hormone levels—free thyroxine (FT4), thyroid‐stimulating hormone (TSH), and thyroid peroxidase antibody (TPO‐Ab)—in the third trimester were collected for analysis of correlations with pregnancy complications and outcomes, using Spearman’s correlation and a logistic regression model. Results: Adverse events, such as premature birth, premature rupture of membranes, placental abruption, fetal distress, fetal growth restriction, neonatal asphyxia, pregnancy‐induced hypertension, gestational diabetes mellitus, and postpartum hemorrhage, occurred among the 646 women with twin pregnancies in the third trimester. The highest proportion of pregnancy outcomes was premature birth, accounting for 42.26%, and the top two pregnancy complications were gestational diabetes mellitus and pregnancy‐induced hypertension, accounting for 31.42% and 20.28%, respectively. Assisted reproduction and twin type correlated with fetal distress during the third trimester of pregnancy and neonatal asphyxia. Maternal age correlated with gestational diabetes mellitus in the third trimester of pregnancy and postpartum hemorrhage. FT4 levels varied in pregnant women with placental abruption, TSH levels varied in pregnant women with pregnancy‐induced hypertension, and TPO‐Ab levels varied in pregnant women with premature birth, placental abruption, and postpartum hemorrhage. Study Limitations: This study only analyzed thyroid hormone levels at a single timepoint (third trimester) during pregnancy. Dynamic changes in hormone levels across different gestational stages—such as TSH suppression patterns in the first trimester due to hCG elevation or FT4 adjustments in the second trimester—were not tracked. This limitation may hinder the understanding of how temporal hormonal variations contribute to complications like gestational diabetes mellitus or placental abruption. For instance, transient TSH fluctuations in early pregnancy or mid‐trimester FT4 instability might differently influence outcomes. Future studies should incorporate longitudinal monitoring at multiple intervals (e.g., first, second, and third trimesters) to capture hormonal trajectories and their phase‐specific associations with adverse outcomes. Additionally, the exclusion of women with preexisting thyroid conditions or medication use limits the generalizability of findings to high‐risk populations who may benefit most from thyroid monitoring. The single‐timepoint design also precludes causal inference—while correlations between third‐trimester hormones and complications are observed, it remains unclear whether these abnormalities initiate pathological processes (e.g., placental dysfunction) or arise as secondary consequences of existing complications. Furthermore, the homogeneity of the cohort (e.g., predominantly dichorionic twins) may underestimate risks in monochorionic pregnancies, which inherently carry higher placental vulnerability. Conclusion: Abnormal changes in thyroid hormones during pregnancy may correlate with adverse events such as pregnancy‐induced hypertension and placental abruption. Clinicians should prioritize routine third‐trimester monitoring of FT4, TSH, and TPO‐Ab levels in twin pregnancies, particularly for women with assisted reproduction or monochorionic placentation, to enable early risk stratification and tailored interventions.