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

Background: Maternal thyroid hormones are essential for fetal development and the maintenance of pregnancy. While thyroid dysfunction earlier in gestation has been extensively studied, the clinical relevance of thyroid function assessed at labor admission remains unclear. This study investigated the association between maternal thyroid function parameters measured at labor ward admission and obstetric and neonatal outcomes in term pregnancies. Methods: In this retrospective observational study, 664 women with singleton term pregnancies (≥37 weeks) admitted to the labor ward of a tertiary referral center were included. Maternal thyroid-stimulating hormone (TSH), free thyroxine (FT4), and admission complete blood count parameters (hemoglobin, hematocrit, white blood cell count, and platelet count) were recorded. Obstetric and neonatal outcomes were compared across FT4 tertiles using univariable and multivariable regression analyses adjusted for key obstetric confounders. Results: Gestational age at delivery differed significantly across FT4 tertiles, with higher FT4 levels associated with a greater proportion of late-term deliveries. Lower FT4 levels were independently associated with lower neonatal birth weight categories after adjustment for gestational age and parity. Admission complete blood count parameters did not differ significantly across FT4 tertiles or gestational age categories. Maternal TSH levels were not independently associated with obstetric or neonatal outcomes, and 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 intrapartum fetal distress or adverse immediate neonatal outcomes.