Correlates of low thyroxine values at newborn screening among infants born before 32 weeks gestation

We assessed the relation of perinatal factors to severe hypothyroxinemia of prematurity, defined as thyroxine value more than 2.6 standard deviations below the mean for newborns. The 365 survivors of birth before 32 weeks gestation were enrolled in a population-based study of the correlates of neonatal brain injury. In this historical cohort study, mothers were interviewed; perinatal data were abstracted from medical records and neonatal data were collected prospectively. Neonatal thyroxine screening values were retrieved from the New Jersey State Department of Health. Associated with severe hypothyroxinemia were: gestational age 23-27 weeks vs. 31 weeks (OR = 5.1, 95% CI 1.7, 15.2), later age at thyroxine test (OR = 1.6 per day, 95% CI 1.2, 2.1), fraction inspired oxygen at age 24 h > 40% (OR = 3.2, 95% CI 1.1, 8.8), mechanical ventilation (OR = 5.1, 95% CI 1.3, 19.4), diastolic blood pressure < 20 mmHg (OR = 2.3, 95% CI 1.2, 4.3), and > 12 years of maternal education (OR = 0.4, 95% CI 0.22, 1.0). Infants with severe hypothyroxinemia had higher mortality, more days of oxygen supplementation, ventilation and hospitalization and were 11 times more likely to require oxygen supplementation at the postnatal age equivalent to 36 weeks gestational age (odds ratio 10.6, 95% CI 2.3, 48.8). In preterm infants, neonatal thyroxine levels obtained at newborn screening in the first week of life may convey important prognostic information about mortality, morbidity, and the risk for bronchopulmonary dysplasia.