Bilirubin is a naturally occurring substance in the body of humans and many other animals, but it is neurotoxic when its concentration in the blood is too high, a condition known as hyperbilirubinemia.

Depending on the level of exposure, the effects range from clinically unnoticeable to severe brain damage and even death.

When hyperbilirubinemia increases past a mild level, it leads to jaundice, raising the risk of progressing to kernicterus.

Mildly elevated serum bilirubin levels are common in newborns, and neonatal jaundice is not unusual, but bilirubin levels must be carefully monitored in case they start to climb, in which case more aggressive therapy is needed, usually via light therapy but sometimes even via exchange transfusion.

These include lethargy, decreased feeding, hypotonia or hypertonia, a high-pitched cry, spasmodic torticollis, opisthotonus, setting sun sign, fever, seizures, and even death.

Clinically, this may result in neurological, learning and movement disorders, isolated hearing loss and auditory dysfunction.

[citation needed] Measuring the serum bilirubin is helpful in evaluating a baby's risk for developing kernicterus.