['Transient hypothyroxinemia (TH) of prematurity has been correlated with poor neurodevelopmental outcome. However, the relationship between thyroid function and neonatal mortality and brain injury has not been described.', 'To investigate the relationship between thyroid function and neonatal outcome.', 'Review of infants weighing <1500 grams admitted to a single level III newborn intensive care unit (n = 342). Serum total T4 values of initial newborn screening of infants dying before hospital discharge were compared with those of surviving infants. Total T4 values from infants with and without intraventricular hemorrhage (IVH) and periventricular leukomalacia (PVL) also were compared.', 'T4 values strongly correlated with gestational age (r = .56). Overall, 289 (85%) of 342 infants had TH. None of the infants had true congenital hypothyroidism. T4 values of infants with cystic PVL (n = 15) were not statistically different from those for infants who did not develop cystic PVL. Infants with IVH (n = 58) had a lower T4 value than did infants who did not develop IVH (5.4 +/- 3.4 microg/dL vs 7.8 +/- 3.6 microg/dL). Infants who died before hospital discharge (n = 24) had a lower T4 value than did infants discharged to home (3.4 +/- 2.2 vs 7.9 +/- 3.7 microg/dL). After controlling for potential confounding variables, T4 value remained associated with an increased odds of IVH (odds ratio: 1.2; 95% confidence interval: 1.05 to 1.4) and mortality (odds ratio: 2.3; 95% confidence interval 1.6 to 3.4).'] Is low serum thyroxine on initial newborn screening associated with intraventricular hemorrhage and death in very low birth weight infants?