Increased mortality in neonates with low thyroxine values.

Serum thyroxine values were determined prospectively over a 6-month interval in 97 newborn infants with birth weight < 2200 gm or gestational age < 37 weeks. There were 89 survivors and 8 deaths. Infants with thyroxine values of < 2.5 micrograms/dl (32.2 nmol/L) had a significantly higher mortality rate (p < 0.001) compared with infants with higher thyroxine levels. In neonates there may exist a critical thyroxine value below which there is an increased risk of mortality.