bilirubins

Bilirubins, commonly referred to simply as bilirubin, are bile pigments formed during heme breakdown in the body's normal turnover of red blood cells. They originate mainly from aged erythrocytes and circulate in the blood bound to albumin in a form called unconjugated or indirect bilirubin.

In the liver, unconjugated bilirubin is taken up by hepatocytes and conjugated with glucuronic acid to form conjugated or direct bilirubin, which is water-soluble. Conjugated bilirubin is then excreted into bile and released into the digestive tract.

Within the intestines, intestinal bacteria convert some conjugated bilirubin to urobilinogen. Most of this is eventually excreted in feces as stercobilin, giving stool its characteristic color; a portion is reabsorbed and may be excreted by the kidneys as urobilinogen.

Clinical relevance centers on bilirubin levels as indicators of liver and biliary function. Total bilirubin measures the sum of direct and indirect bilirubin. Normal adult total bilirubin is typically about 0.3 to 1.2 mg/dL, with direct bilirubin usually less than 0.3 mg/dL. Elevated levels can indicate different patterns: prehepatic (increased indirect bilirubin due to hemolysis), hepatic (liver dysfunction affecting conjugation or excretion), or posthepatic (bile duct obstruction leading to increased direct bilirubin).

In newborns, jaundice from elevated unconjugated bilirubin is common because of immature UDP-glucuronosyltransferase activity; severe cases can risk kernicterus if untreated. Bilirubin levels are commonly assessed as part of liver function testing and can guide clinical assessment of jaundice and related conditions.