Bilirubin metabolism and jaundice: pre-hepatic, hepatic, post-hepatic
Jaundice is one sign with three places to fail. Follow bilirubin from haem to the gut, then use the fraction, the enzymes, the urine and the stool to find where the pathway breaks.
Chapters (11)
Trace bilirubin from haem breakdown through conjugation and excretion, and classify jaundice as pre-hepatic, hepatic or post-hepatic.
The short version
- Haem becomes biliverdin, then unconjugated bilirubin, which travels on albumin; the liver conjugates it, bile carries it out, and gut bacteria make urobilinogen.
- Only conjugated bilirubin appears in the urine, because unconjugated bilirubin is water-insoluble and albumin-bound.
- Pre-hepatic jaundice is unconjugated, with normal enzymes and no bilirubin in the urine.
- Hepatic jaundice can raise either fraction or both, depending on whether uptake, conjugation, excretion or hepatocyte injury is the problem.
- Post-hepatic jaundice is conjugated, with a high alkaline phosphatase, often itching, and pale stool when the block is complete.
Where in the bilirubin pathway is the problem?
A 21-year-old student looks slightly yellow in exam week, after a day with hardly any food. His total bilirubin is 3.1 mg/dL, his liver enzymes are normal, and he is not anaemic. Where in the bilirubin pathway could the problem be?
This lesson covers bilirubin metabolism and jaundice: the steps from haem breakdown to excretion, and the three-way classification that tells you which step has failed. Jaundice becomes visible when bilirubin reaches about 2–3 mg/dL.[1][6]
Bilirubin metabolism: from haem to the gut
Follow the pathway one step at a time. Each step has a typical failure, and the classification later in the lesson depends on it.
- 1. ProductionOld red cells are broken down, mostly in the spleen. Haem oxygenase turns haem into biliverdin, and biliverdin reductase turns biliverdin into bilirubin. About 250–350 mg forms each day, and 70–80% comes from old red cells.[2][4]
- 2. TransportThis bilirubin is unconjugated, so it does not dissolve in water. It travels bound to albumin, which keeps it out of the urine.[2][4]
- 3. Conjugation in the liverThe liver takes bilirubin up, and the enzyme glucuronyl transferase (UGT1A1) attaches glucuronic acid. Now it is conjugated, water-soluble and ready to be pumped into bile.[2][4]
- 4. The gutGut bacteria turn bilirubin into urobilinogen, and most of that becomes stercobilin, which makes stool brown. Some urobilinogen is reabsorbed and recycled by the liver, and a little reaches the urine.[2][7][8]
Classifying jaundice by where the pathway fails
Classify jaundice by the step that fails: too much production before the liver, a fault inside the liver, or blocked bile flow after it.[1]
| Type | Problem | Bilirubin | Typical findings |
|---|---|---|---|
| Pre-hepatic | Too much production, as in haemolysis | Unconjugated; usually below 3.5 mg/dL | No bilirubin in the urine; normal liver enzymes |
| Hepatic | Uptake, conjugation or excretion inside the liver is faulty | Either form, or both | Depends on the cause; see below |
| Post-hepatic | Blocked bile flow (stone, pancreatic tumour) | Conjugated | Bilirubin in the urine, high alkaline phosphatase; pale stool and itching, especially if the block is complete |
Pre-hepatic jaundice: too much production
In haemolysis, bilirubin is made faster than the liver can take it up and conjugate it, so the excess is unconjugated. A normal liver still clears most of the extra load, so the level usually stays below 3.5 mg/dL. The urine has no bilirubin, and the liver enzymes are normal.[1]
The absence of anaemia separates Gilbert syndrome from haemolysis, which is why the opening student does not fit this group.[3]
Hepatic jaundice: Gilbert, Crigler-Najjar, hepatitis, Dubin-Johnson and Rotor
Hepatic jaundice means uptake, conjugation or excretion inside the liver is faulty. The cause decides which fraction rises.[1][3][5]
| Cause | What fails | Bilirubin and enzymes |
|---|---|---|
| Gilbert syndrome | Mild UGT1A1 defect; worsens with fasting | Mild unconjugated rise; normal liver tests otherwise |
| Crigler-Najjar | UGT1A1 deficiency: complete and severe in type 1, partial and milder in type 2 | Unconjugated |
| Hepatitis (hepatocellular injury) | Hepatocytes are injured | Both forms rise; transaminases are high |
| Dubin-Johnson | Impaired excretion of conjugated bilirubin | Conjugated; normal enzymes; no cholestasis |
| Rotor | Impaired reuptake of conjugated bilirubin | Conjugated; normal enzymes; no cholestasis |
Gilbert syndrome is the pattern of the opening student: a mild UGT1A1 defect that worsens with fasting, with normal enzymes and no anaemia.[3]
Post-hepatic jaundice: blocked bile flow
Post-hepatic jaundice means blocked bile flow, from a stone or a pancreatic tumour. Conjugated bilirubin backs up into the blood and the urine, alkaline phosphatase rises, the stool turns pale and the skin itches.[1]
In complete obstruction no bilirubin reaches the gut, so no urobilinogen forms.[1][2][7]
Three patients, three patterns
Put the classification to work. Each patient below has a different pattern.[1]
| Findings | Pattern |
|---|---|
| Total 2.6 mg/dL, nearly all unconjugated; low haemoglobin; normal enzymes | Haemolysis (pre-hepatic) |
| Total 8 mg/dL, both forms raised; transaminases 900 U/L | Hepatocellular (hepatic) |
| Total 12 mg/dL, mostly conjugated; alkaline phosphatase 600 U/L; pale stools | Obstruction (post-hepatic) |
Practice question
Original · USMLE styleA 58-year-old woman has had 2 weeks of progressive jaundice and itching, and her stools have become clay-coloured. Laboratory studies show:
| Total bilirubin | 17.6 mg/dL |
|---|---|
| Direct bilirubin | 15.2 mg/dL |
| Alkaline phosphatase | 840 U/L |
| AST | 72 U/L |
| ALT | 66 U/L |
Common questions
What is the difference between conjugated and unconjugated bilirubin?
Unconjugated bilirubin is not water-soluble and travels bound to albumin. After the liver attaches glucuronic acid with UGT1A1, it is conjugated and water-soluble, ready to be pumped into bile.
Why does only conjugated bilirubin appear in the urine?
Unconjugated bilirubin is water-insoluble and albumin-bound, so it does not reach the urine. Conjugated bilirubin is water-soluble, so when it backs up into the blood it appears in the urine.
How do you tell pre-hepatic, hepatic and post-hepatic jaundice apart?
Use the bilirubin fraction, the liver enzymes, the urine and the stool. Pre-hepatic is unconjugated with normal enzymes, post-hepatic is conjugated with high alkaline phosphatase, and pale stool when the block is complete, and hepatic can be either or both.
At what bilirubin level does jaundice become visible?
Jaundice becomes visible when bilirubin reaches about 2–3 mg/dL.
Transcript
Read the full narration, by chapter669 words
0:00A fasting student with yellow eyes
A 21-year-old student looks slightly yellow in exam week, after a day with hardly any food. His total bilirubin is 3.1 milligrams per decilitre, his liver enzymes are normal, and he is not anaemic. Where in the bilirubin pathway could the problem be? Today: trace bilirubin from haem breakdown to excretion, and classify jaundice as pre-hepatic, hepatic or post-hepatic.
0:26When jaundice shows, and where bilirubin comes from
Jaundice becomes visible when bilirubin reaches about 2 to 3 mg/dL. Step one, production. Old red cells are broken down, mostly in the spleen, and haem oxygenase turns haem into biliverdin, which biliverdin reductase turns into bilirubin. About 250 to 350 milligrams form each day, and 70 to 80% comes from old red cells. This bilirubin is unconjugated, so it does not dissolve in water.
1:00Transport on albumin and conjugation in the liver
Step two, transport: it travels bound to albumin, which keeps it out of the urine. Step three, the liver takes it up, and the enzyme glucuronyl transferase, UGT1A1, attaches glucuronic acid. Now it is conjugated, water-soluble, and ready to be pumped into bile.
1:21Gut bacteria, urobilinogen and the urine rule
Step four, in the gut, bacteria turn it into urobilinogen, and most of that becomes stercobilin, which makes stool brown. Some urobilinogen is reabsorbed and recycled by the liver, and a little reaches the urine. One rule to hold on to: only conjugated bilirubin appears in the urine. Now classify jaundice by where the pathway
1:42Pre-hepatic jaundice: too much production
fails. Pre-hepatic means too much production, as in haemolysis. The liver copes, so the extra bilirubin is unconjugated, usually below three and a half mg/dL. The urine has no bilirubin, and the liver enzymes are normal.
2:01Hepatic jaundice: Gilbert, Crigler-Najjar, hepatitis, Dubin-Johnson, Rotor
Hepatic means uptake, conjugation or excretion inside the liver is faulty. Gilbert syndrome is a mild UGT1A1 defect that worsens with fasting, which is our student. Crigler-Najjar is a UGT1A1 deficiency: complete and severe in type one, partial and milder in type two. Hepatitis injures hepatocytes, so both forms rise, and the transaminases are high. Dubin-Johnson and Rotor syndromes raise conjugated bilirubin, from impaired excretion and impaired reuptake respectively, with normal enzymes and no cholestasis.
2:42Post-hepatic jaundice: blocked bile flow
Post-hepatic means blocked bile flow, from a stone or a pancreatic tumour. Conjugated bilirubin backs up into the blood and the urine, alkaline phosphatase rises, the stool turns pale, and the skin itches. In complete obstruction no bilirubin reaches the gut, so no urobilinogen forms. Try three patients.
3:04Three patients, three patterns
The first has a total of 2.6, nearly all unconjugated, low haemoglobin and normal enzymes. That is haemolysis. The second has a total of eight, both forms raised, and transaminases of 900. That is hepatocellular. The third has a total of 12, mostly conjugated, alkaline phosphatase of 600, and pale stools. That is obstruction. Your turn.
3:34Practice question
A 58-year-old woman has had two weeks of progressive jaundice and itching, and her stools have become clay-coloured. Total bilirubin is 17.6 milligrams per decilitre, with a direct fraction of 15.2. Alkaline phosphatase is 840 units per litre, AST 72, and ALT 66. CT shows a mass in the head of the pancreas with dilated bile ducts. Which of the following urine findings is most likely? A, bilirubin present, urobilinogen absent. B, bilirubin present, urobilinogen increased. C, bilirubin absent, urobilinogen absent. D, bilirubin absent, urobilinogen increased. E, bilirubin absent, urobilinogen normal. Pause here and decide.
4:33Answer: wrong options first, then the right one
B is out: raised urobilinogen needs bilirubin to reach the gut, and her clay-coloured stools say it is not arriving. C is out: conjugated hyperbilirubinaemia spills bilirubin into the urine. D is out: a blocked duct cannot raise urobilinogen. E is out: that is the pattern of an unconjugated problem such as Gilbert syndrome. So the answer is A. A blocked duct lets conjugated bilirubin back up into the urine, while none reaches the gut to make urobilinogen.
5:09Recap and next lesson
Three things to keep. One: haem becomes biliverdin, then unconjugated bilirubin on albumin; the liver conjugates it, bile carries it out, and gut bacteria make urobilinogen. Two: pre-hepatic is unconjugated, hepatic can be either or both, and post-hepatic is conjugated with pale stool and high alkaline phosphatase. Three: only conjugated bilirubin reaches the urine. Next, liver function test patterns.
Sources
Numbers in the text, e.g. [1], point here- MSD Manual Professional Version. Jaundice (Aug 2025). msdmanuals.com/professional/hepatic-and-biliary-disorders/approach-to-the-patient-with-liver-disease/jaundice
- MSD Manual Professional Version. Liver Structure and Function: overview of bilirubin metabolism. msdmanuals.com/professional/hepatic-and-biliary-disorders/approach-to-the-patient-with-liver-disease/liver-structure-and-function
- MSD Manual Professional Version. Inborn Metabolic Disorders Causing Hyperbilirubinemia (Mar 2026). msdmanuals.com/professional/hepatic-and-biliary-disorders/approach-to-the-patient-with-liver-disease/inborn-metabolic-disorders-causing-hyperbilirubinemia
- Bilirubin Hepatic and Intestinal Transport and Catabolism, 2025 review. ebi.ac.uk/europepmc/webservices/rest/PMC12649357/fullTextXML
- Dubin-Johnson and Rotor syndromes review, 2026. ebi.ac.uk/europepmc/webservices/rest/PMC13247717/fullTextXML
- Ocular Clues to Liver Disease, 2026. ebi.ac.uk/europepmc/webservices/rest/PMC13205696/fullTextXML
- Europe PMC article on bilirubin conversion to urobilinogen by gut bacteria, 2026. ebi.ac.uk/europepmc/webservices/rest/PMC13102838/fullTextXML
- OpenStax Anatomy & Physiology 2e, 23.6 Accessory Organs in Digestion: The Liver, Pancreas, and Gallbladder. openstax.org/books/anatomy-and-physiology-2e/pages/23-6-accessory-organs-in-digestion-the-liver-pancreas-and-gallbladder
The practice question is original and its patient is synthetic. Every fact is checked against the sources above in independent accuracy reviews. Always follow your local guidelines and your own laboratory’s ranges.
Image credits. Illustrations: Servier Medical Art (smart.servier.com), licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/), adapted (recoloured, outlined, cropped, resized; the eye's white tinted yellow by an overlay): eye, erythrocyte, spleen, liver and gallbladder, bile secretion (hepatocytes), duodenum, colon, urine specimen container, biliary lithiasis, pancreas. Original diagrams drawn in code: lab slips, the haem to bilirubin schematic, albumin carriers, the UGT1A1 step, colour chips, the question card and recap card. No AI-generated teaching images.
Corrections. None so far. Spotted an error? Email info@medservatory.com with the lesson code MS-S1-GI-003.
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