bilirubiinin

Bilirubiinin is a bile pigment formed from the breakdown of heme, primarily from aged red blood cells. It exists in two major forms: unconjugated (indirect) bilirubin, which is lipid-soluble and travels in the bloodstream bound to albumin, and conjugated (direct) bilirubin, which is water-soluble after being processed in the liver.

In the liver, hepatocytes take up unconjugated bilirubin and conjugate it with glucuronic acid through the enzyme UDP-glucuronosyltransferase (UGT1A1) to form bilirubin diglucuronide. This conjugated bilirubin is excreted into bile and delivered to the intestine. In the gut, bacteria can deconjugate some bilirubin; most is reduced to urobilinogen. Some urobilinogen is reabsorbed and excreted in urine as urobilin, while most of the remainder becomes stercobilin, which gives stool its color.

Elevations in bilirubiinin or its fractions can cause jaundice. Prehepatic (hemolytic) causes raise unconjugated bilirubin; hepatic diseases can impair uptake, conjugation, or secretion; posthepatic (obstructive) causes raise conjugated bilirubin due to bile duct obstruction. In newborns, physiologic jaundice is common because UGT1A1 activity is immature; breast-feeding jaundice and other neonatal conditions can contribute.

Laboratory evaluation typically measures total bilirubin and direct (conjugated) bilirubin to determine the indirect fraction. Management targets the underlying cause; severe neonatal hyperbilirubinemia may require phototherapy or exchange transfusion, while adults are treated according to the disease driving bilirubin elevation.