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[Biochemistry](https://durgical.com/en/biochemistry/) · Molecule

# Bilirubin

Bilirubin is the yellow pigment left when heme is broken down; when it builds up, it turns skin and eyes yellow (jaundice).

Updated 2026-11-01

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## Key facts

- Normal total bilirubin in adults: up to about 1.2 mg/dL; direct (conjugated) below 0.3 mg/dL.
- Jaundice becomes visible when total bilirubin exceeds about 2–3 mg/dL; first in the sclerae.
- Unconjugated (indirect): fat-soluble, travels on albumin, does not appear in urine and crosses the blood–brain barrier.
- Conjugated (direct): water-soluble; when it builds up in blood it appears in urine (dark urine).

**Formula**
C₃₃H₃₆N₄O₆

**PubChem**
[CID 5280352](https://pubchem.ncbi.nlm.nih.gov/compound/5280352)

## Structure

- A linear tetrapyrrole (the heme ring opened); internal hydrogen bonds fold it and hide its polar groups, which is why it is insoluble.
- Blue light (phototherapy) converts it into isomers that cannot fold and are excreted without conjugation.

## Clinical notes

- Indirect hyperbilirubinemia: hemolysis, Gilbert, Crigler-Najjar, neonatal jaundice. Direct: hepatitis, cholestasis, biliary obstruction, Dubin-Johnson, Rotor.
- Free bilirubin in newborns can stain the basal ganglia (kernicterus); some drugs (sulfonamides, ceftriaxone) displace it from albumin.
- Delta bilirubin, covalently bound to albumin, lasts for weeks and explains why jaundice fades slowly after an obstruction is relieved.
- Pigment (black) gallstones are made of calcium bilirubinate and are typical of chronic hemolysis.

## Pathways

- [Heme breakdown and bilirubin](https://durgical.com/en/biochemistry/pathways/heme-breakdown-and-bilirubin/)

## Related conditions

- [Gilbert syndrome](https://durgical.com/en/conditions/gilbert-syndrome/)
- [Crigler-Najjar syndrome](https://durgical.com/en/conditions/crigler-najjar-syndrome/)
- [Dubin-Johnson syndrome](https://durgical.com/en/conditions/dubin-johnson-syndrome/)
- [Rotor syndrome](https://durgical.com/en/conditions/rotor-syndrome/)
- [Neonatal jaundice](https://durgical.com/en/conditions/neonatal-jaundice/)
- [Gallstones (cholelithiasis)](https://durgical.com/en/conditions/gallstones/)
- [Viral hepatitis](https://durgical.com/en/conditions/viral-hepatitis/)

## Related

- [Heme](https://durgical.com/en/biochemistry/molecules/heme/)
- [Heme oxygenase](https://durgical.com/en/biochemistry/enzymes/heme-oxygenase/)
- [Biliverdin reductase](https://durgical.com/en/biochemistry/enzymes/biliverdin-reductase/)
- [UGT1A1 (bilirubin UDP-glucuronosyltransferase)](https://durgical.com/en/biochemistry/enzymes/ugt1a1/)
- [Hemoglobin and 2,3-BPG](https://durgical.com/en/biochemistry/enzymes/hemoglobin-and-2-3-bpg/)

## Test yourself

Think of the answer, then tap the question to see it.

## Which bilirubin fraction can appear in urine?

Only conjugated (direct) bilirubin, because it is water-soluble and not bound to albumin.

## Why can unconjugated bilirubin cause kernicterus?

It is fat-soluble; the free fraction crosses the immature blood–brain barrier and deposits in the basal ganglia.

## How does phototherapy act on bilirubin?

Blue light turns it into more soluble isomers that leave in bile and urine without conjugation.

## At what level does jaundice usually become visible?

When total bilirubin exceeds about 2–3 mg/dL, first in the sclerae.

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Molecules are shown in their neutral form, as the databases deposit them; in the cell the acids and phosphates are ionised.

Educational information, not medical advice. For any symptom, see a health professional.

Sources: [Bilirubin (CID 5280352)](https://pubchem.ncbi.nlm.nih.gov/compound/5280352) (PubChem, NCBI, 2026) · Public domain; [Bilirubin blood test: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/003479.htm) (MedlinePlus, US National Library of Medicine, 2026); [Jaundice | Icterus | MedlinePlus](https://medlineplus.gov/jaundice.html) (MedlinePlus, US National Library of Medicine, 2026); Textbook of Biochemistry for Medical Students, 6th ed. (Jaypee Brothers Medical Publishers (Vasudevan DM, Sreekumari S, Vaidyanathan K), 2011).

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