It results from an accelerated hemolysis and an immature hepatic system for the uptake, conjugation and secretion of bilirubin. The UDP-glucuronosyltransferase activity is reduced. Since the increased bilirubin is unconjugated, it is capable of penetrating the blood-brain barrier when its concentration exceeds that which can be tightly bound to albumin. This can result in a hyperbilirubinemic toxic encephalopathy, or kernicterus, which can cause mental retardation. Phenobarbital can induce the bilirubin metabolizing system and is effective in treating neonatal jaundice.
Phototherapy can promote the excretion of unconjugated bilirubin by an unknown process.