Thyroid-Stimulating Hormone and Free Thyroxine Levels at Labor Admission: Associations with Obstetric and Neonatal Outcomes in Term Pregnancies

Background: Maternal thyroid hormones play an essential role in fetal growth and pregnancy maintenance. While thyroid dysfunction earlier in pregnancy has been widely studied, the clinical relevance of thyroid function assessed at the time of labor admission remains unclear. This study aimed to evaluate the association between maternal thyroid function parameters measured at labor ward admission and obstetric and neonatal outcomes in term pregnancies. Methods: This retrospective observational study included 664 women with singleton term pregnancies (≥37 weeks) admitted to the labor ward of a tertiary referral center between May 2020 and May 2025. Maternal thyroid-stimulating hormone (TSH) and free thyroxine (FT4) levels were measured at admission. Obstetric outcomes included gestational age at delivery and mode of delivery, while neonatal outcomes comprised birth weight, Apgar scores, and neonatal intensive care unit (NICU) admission. Univariable analyses and multivariable logistic and ordinal regression models adjusted for key obstetric confounders were performed. Results: Gestational age at delivery differed significantly across FT4 tertiles, with higher FT4 levels associated with a greater proportion of late-term deliveries (p = 0.018). Lower FT4 levels were independently associated with lower neonatal birth weight categories after adjustment for gestational age and parity (adjusted OR per 1 SD decrease: 1.19; 95% CI: 1.02–1.38; p = 0.025). Although an inverse association between FT4 tertiles and cesarean delivery due to fetal distress was observed in unadjusted analyses, this association was not retained after multivariable adjustment. Maternal TSH levels were not independently associated with obstetric or neonatal outcomes. No significant associations were observed with Apgar scores or NICU admission. Conclusions: In term pregnancies, maternal FT4 levels measured at labor admission are associated with delivery timing and neonatal birth weight but do not independently predict cesarean delivery due to fetal distress or adverse immediate neonatal outcomes.