Comparative Analysis of Thyroid Function Tests in Neonatal Intensive Care Unit Patients and Healthy Newborns

This study investigated the relationship between thyroid function and key perinatal factors, including mode of delivery, gestational age, and clinical condition, by comparing 152 neonates hospitalized in the neonatal intensive care unit (NICU) with 50 healthy newborns. Thyroid function tests (capillary thyroid-stimulating hormone (cTSH), venous TSH (vTSH), and free T4) were evaluated according to gestational age, delivery mode, and inferred exposure to povidone-iodine used for cesarean skin preparation. Serum free T4 concentrations were significantly lower in preterm and NICU infants compared with term controls (p = 0.007), while cTSH and vTSH levels were not significantly different. Among NICU infants, those delivered by cesarean section had lower free T4 levels than those born vaginally (p = 0.039), and free T4 was particularly reduced in infants with sepsis (p = 0.022). A moderate correlation between cTSH and vTSH was observed (ρ = 0.50, p < 0.001). These findings indicate that lower free T4 levels in preterm and clinically ill neonates likely represent transient, adaptive responses rather than permanent hypothyroidism, and that differences by delivery mode may reflect a combination of perinatal factors and inferred iodine exposure.