Thyroid Function in Term and Late Preterm Infants with Respiratory Distress in Relation to Severity of Illness

Background: Transient hypothyroxinemia is common in extremely premature infants, but has not been extensively investigated in ill term and late preterm infants. We hypothesized that free thyroxine (T(4)) levels in term and late preterm infants with respiratory distress would be inversely related to severity of illness Methods: Eligible infants included those had >= 35 weeks of gestation requiring mechanical ventilation or nasal continuous positive airway pressure. Thyroid function (thyroid-stimulating hormone, T(4), free T(4) [fT(4)], and free triiodothyronine) and cortisol levels were collected at four specified intervals (birth, day 2, day 3, and day 5 of life). Illness severity was measured using score for neonatal acute physiology (SNAP). Infants with an SNAP >= 10 were considered to have severe illness. Statistical analysis included repeated measures analysis of variance. Results: A total of 20 patients were enrolled in the study. The mean gestational age of study infants was 37.0 +/- 1.7 weeks. Infant with SNAP >= 10 had a diminished thyroid-stimulating hormone at birth. There was an inverse correlation between SNAP at birth and fT(4). There was also an inverse correlation between lowest measured fT(4) and highest mean airway pressure, oxygenation index, and A-a gradient. Infants who received inhaled nitric oxide had diminished fT(4) levels compared to infants who did not receive this therapy. There were no differences in cortisol in those infants with and without SNAP >= 10 during the study period. Conclusions: In our study sample, there was an association between severity of illness and fT(4) levels at birth, and infants requiring inhaled nitric oxide showed decreased fT(4) over the study time. Further research is warranted to determine whether T(4) supplementation would be beneficial in term and late preterm infants with respiratory distress.