Unconjugated (indirect) bilirubin is the fat-soluble form produced from haemoglobin breakdown before the liver has processed it. It travels through the blood bound to albumin, is taken up by the liver, and converted to conjugated bilirubin for excretion.

Elevated unconjugated bilirubin indicates either excessive red blood cell destruction (haemolysis) — producing more bilirubin than the liver can process — or impaired hepatic uptake and conjugation. Gilbert's syndrome, a benign inherited condition affecting approximately 5–10% of the population, causes mildly elevated unconjugated bilirubin due to reduced UDP-glucuronosyltransferase enzyme activity, often noticed as mild jaundice during fasting, illness, or alcohol intake.