Liver Functions & Bilirubin Metabolism

The liver is the body's central metabolic factory and detoxification organ. For NEET PG, the highest-yield slice of this topic is bilirubin metabolism and the classification of jaundice — differentiating pre-hepatic, hepatic and post-hepatic causes using bilirubin fractions, urine bile pigments and stool colour is a perennial favourite. This note builds the metabolic foundation first, then drills the jaundice work-up.

Overview of hepatic functions

The hepatocyte performs an astonishing range of tasks. Group them for recall:

• Metabolic — glucose homeostasis (glycogenesis, glycogenolysis, gluconeogenesis), lipid metabolism (β-oxidation, lipoprotein and cholesterol synthesis, ketogenesis), protein metabolism (deamination, transamination, urea cycle).
• Synthetic — albumin, all clotting factors except factor VIII and von Willebrand factor, complement, angiotensinogen, thrombopoietin, most binding/carrier proteins.
• Storage — glycogen, vitamins A, D, B12, iron (as ferritin), copper.
• Detoxification & excretion — ammonia → urea, drug metabolism via cytochrome P450, bilirubin conjugation, hormone and bile acid clearance.
• Immune — Kupffer cells (resident macrophages) filter portal blood.

High-yield: Factor VIII and vWF are NOT made by the hepatocyte (VIII is made by sinusoidal endothelium/extrahepatic sites, vWF by endothelium and megakaryocytes). Hence factor VIII may be normal or raised in liver failure, which helps distinguish hepatic coagulopathy from DIC, where factor VIII falls.

Glucose metabolism

The liver is the glucostat. Postprandially it stores glucose as glycogen and converts excess to fat; during fasting it releases glucose by glycogenolysis (first ~24 h) and then gluconeogenesis from lactate, glycerol and glucogenic amino acids (alanine via the Cahill cycle). Only the liver and kidney express glucose-6-phosphatase, so only they can export free glucose into blood.

Protein and ammonia handling

The urea cycle is unique to the liver and is the principal route for disposing of nitrogen. When hepatocytes fail, ammonia accumulates → hepatic encephalopathy. Albumin (half-life ~20 days) reflects chronic synthetic function; the prothrombin time/INR, depending on short-lived factors (especially factor VII, half-life ~6 h), is the earliest and most sensitive marker of acute synthetic failure.

Detoxification and cytochrome P450

Drug metabolism proceeds in two phases:

Phase	Reaction type	Enzymes	Example
Phase I	Oxidation/reduction/hydrolysis (makes drug more polar)	Cytochrome P450 (CYP) family — CYP3A4 most abundant	Paracetamol → NAPQI
Phase II	Conjugation (glucuronidation, sulphation, acetylation, glutathione)	UGT, sulphotransferases, GST	Bilirubin glucuronidation; NAPQI detox by glutathione

High-yield: Paracetamol toxicity overwhelms glutathione, leaving toxic NAPQI — antidote is N-acetylcysteine (replenishes glutathione). CYP inducers (rifampicin, phenytoin, carbamazepine, phenobarbitone, chronic alcohol, griseofulvin — mnemonic "CRAP-GPS") speed metabolism; inhibitors (cimetidine, ketoconazole, erythromycin, isoniazid, grapefruit juice, ritonavir, sodium valproate) raise drug levels.

Bile and bile acids

Bile is the vehicle for cholesterol excretion and fat digestion. Primary bile acids — cholic acid and chenodeoxycholic acid — are synthesised from cholesterol, the rate-limiting enzyme being 7α-hydroxylase (CYP7A1). In the gut, bacteria convert them to secondary bile acids (deoxycholic and lithocholic acid). About 95% are reabsorbed in the terminal ileum and recycled — the enterohepatic circulation (6–8 cycles/day). Loss of terminal ileum (resection, Crohn's) → bile acid malabsorption, steatorrhoea and increased risk of pigment/cholesterol gallstones.

High-yield: Bile salts emulsify fat and form micelles, essential for absorption of fat-soluble vitamins (A, D, E, K). Obstructive jaundice → fat malabsorption → vitamin K deficiency → prolonged PT that corrects with parenteral vitamin K (unlike hepatocellular failure, which does not correct).

Bilirubin metabolism — the core

Bilirubin is the breakdown product of haem. Roughly 80% comes from senescent red cell haemoglobin; the remainder from ineffective erythropoiesis and other haemproteins (myoglobin, cytochromes, catalase). Total daily production is ~250–350 mg.

Stepwise pathway

Haem → biliverdin → unconjugated bilirubin → albumin transport → hepatic uptake → conjugation → bile excretion → urobilinogen → stercobilin/urobilin.

1. Haem oxygenase (in reticuloendothelial macrophages of spleen, liver, marrow) opens the haem ring → biliverdin + carbon monoxide (CO) + iron. (Endogenous CO production is a clue — exhaled CO reflects haem turnover.)
2. Biliverdin reductase converts green biliverdin → yellow unconjugated bilirubin (UCB).
3. UCB is lipid-soluble, water-insoluble, so it travels in plasma bound to albumin. It is not filtered by the kidney → never appears in urine.
4. At the hepatocyte, UCB is taken up (by OATP) and bound to ligandin (Y protein).
5. Conjugation: UDP-glucuronosyl transferase (UGT1A1) adds glucuronic acid → bilirubin diglucuronide (conjugated, CB), now water-soluble.
6. CB is actively secreted into bile canaliculi by MRP2 — this is the rate-limiting step of the whole pathway.
7. In the gut, bacteria deconjugate and reduce CB → urobilinogen (colourless).
8. Most urobilinogen → stercobilin (gives stool its brown colour) and is excreted in faeces. ~10–20% is reabsorbed (enterohepatic), and a small fraction reaches the kidney → oxidised to urobilin (gives urine its yellow colour).

Feature	Unconjugated (indirect)	Conjugated (direct)
Water solubility	Insoluble (lipid-soluble)	Soluble
Albumin binding	Tightly bound	Loosely bound
Crosses BBB	Yes → kernicterus risk	No
Renal excretion	Never (not filtered)	Yes (dark urine)
Van den Bergh reaction	Indirect (needs methanol)	Direct (immediate)

High-yield: The van den Bergh reaction — direct positive = conjugated bilirubin (reacts immediately with diazo reagent); indirect positive = unconjugated (needs alcohol/methanol). Biphasic in mixed pictures.

High-yield: Only conjugated bilirubin appears in urine. Therefore bilirubinuria always indicates conjugated hyperbilirubinaemia (hepatic or obstructive), never pure haemolysis.

Jaundice (icterus)

Clinically detectable when serum bilirubin exceeds ~2–2.5 mg/dL (sclera first, because of high elastin affinity for bilirubin). Normal total bilirubin is ~0.3–1.2 mg/dL.

Classification flow

Is the rise unconjugated or conjugated? → If unconjugated: think over-production (haemolysis) or under-conjugation (Gilbert, Crigler-Najjar, neonatal). → If conjugated: think hepatocellular injury or biliary obstruction → use urine, stool and enzymes to localise.

Parameter	Pre-hepatic (haemolytic)	Hepatic (hepatocellular)	Post-hepatic (obstructive)
Predominant bilirubin	Unconjugated	Mixed	Conjugated
Urine bilirubin	Absent	Present	Present (markedly)
Urine urobilinogen	Increased	Variable/normal	Decreased/absent
Stool colour	Dark (normal/↑ stercobilin)	Normal/pale	Pale/clay-coloured
ALT/AST	Normal	Markedly raised	Mildly raised
ALP/GGT	Normal	Mild rise	Markedly raised
Other clues	↑ Reticulocytes, ↓ haptoglobin, ↑ LDH	↑ INR not correcting with vitamin K	↑ INR corrects with vitamin K; pruritus, steatorrhoea

High-yield: In complete biliary obstruction, no bilirubin reaches the gut → no urobilinogen formed → urine urobilinogen is absent and stools are clay-coloured, while urine is dark (conjugated bilirubin spilling over). This combination — dark urine + pale stool + absent urinary urobilinogen — is the classic obstructive triad.

High-yield: In haemolysis, increased bilirubin load → increased urobilinogen → raised urinary urobilinogen but no bilirubinuria (acholuric jaundice), because the excess is unconjugated and unfilterable.

Enzyme patterns

• Hepatocellular pattern: AST/ALT rise out of proportion to ALP. AST:ALT ratio >2 suggests alcoholic liver disease ("Scotch = AST"); ALT > AST in viral hepatitis and NAFLD.
• Cholestatic pattern: ALP and GGT rise disproportionately. A raised ALP with raised GGT confirms hepatic origin (rather than bone).

Inherited hyperbilirubinaemias

Disorder	Defect	Bilirubin type	Notes
Gilbert syndrome	↓ UGT1A1 activity (~30%)	Unconjugated (mild)	Common, benign; jaundice on fasting/stress/illness; no treatment
Crigler-Najjar type I	Absent UGT1A1	Unconjugated (severe)	Kernicterus; fatal without liver transplant; no response to phenobarbitone
Crigler-Najjar type II (Arias)	Markedly ↓ UGT1A1	Unconjugated (moderate)	Responds to phenobarbitone (enzyme inducer)
Dubin-Johnson	Defective MRP2 (canalicular export)	Conjugated	Black liver (pigment); normal cholangiogram; benign
Rotor syndrome	Defective hepatic storage/uptake (OATP1B1/3)	Conjugated	Liver not pigmented; benign

High-yield: Gilbert → episodic unconjugated jaundice precipitated by fasting. Crigler-Najjar type II responds to phenobarbitone, type I does not. Dubin-Johnson = black liver + conjugated bilirubinuria.

Neonatal jaundice quick pointers

Physiological jaundice appears after 24 h, peaks day 3–5, unconjugated, due to immature UGT, high RBC turnover and increased enterohepatic circulation. Pathological if it appears <24 h, total >15 mg/dL, conjugated fraction high, or persists >2 weeks. Treatment: phototherapy (converts UCB to water-soluble lumirubin/photoisomers) and exchange transfusion in severe cases. Kernicterus = unconjugated bilirubin crossing the immature BBB, depositing in basal ganglia (globus pallidus, subthalamic nucleus).

Diagnosis & investigation of choice

1. Fractionate bilirubin (conjugated vs unconjugated) → first branch point.
2. LFTs — ALT/AST, ALP, GGT, albumin, PT/INR.
3. Urine — bilirubin and urobilinogen.
4. Haemolysis screen if unconjugated — reticulocytes, peripheral smear, LDH, haptoglobin, Coombs.
5. Imaging for obstruction:
   • Ultrasound abdomen = first-line, best initial test to detect dilated ducts and gallstones.
   • MRCP = investigation of choice (non-invasive) to define the level/cause of biliary obstruction.
   • ERCP = both diagnostic and therapeutic (stone removal, stenting) — preferred when intervention is anticipated.

High-yield: Ultrasound is the best initial imaging in obstructive jaundice; MRCP is the best non-invasive test to delineate ductal anatomy; ERCP is best when therapy (stone extraction/stenting) is also needed.

Management / drug-of-choice highlights

• Haemolytic (pre-hepatic): treat the underlying cause; folate supplementation; transfusion if severe.
• Crigler-Najjar II / Gilbert symptomatic: phenobarbitone (induces UGT1A1).
• Pruritus of cholestasis: cholestyramine (bile-acid binding resin); rifampicin or naltrexone second line.
• Obstructive (stones): ERCP with sphincterotomy/stone extraction; surgery for malignant obstruction or stenting.
• Paracetamol-induced hepatic injury: N-acetylcysteine.
• Vitamin K parenterally in cholestasis-related coagulopathy.
• Hepatic encephalopathy: lactulose + rifaximin (lower ammonia).

Complications

• Kernicterus / bilirubin encephalopathy from unconjugated hyperbilirubinaemia in neonates.
• Coagulopathy and bleeding from reduced clotting-factor synthesis and vitamin K malabsorption.
• Fat-soluble vitamin deficiencies and steatorrhoea in chronic cholestasis.
• Pigment gallstones in chronic haemolysis (calcium bilirubinate); cholesterol stones in bile-acid loss.
• Ascending cholangitis (Charcot triad: fever, RUQ pain, jaundice) in obstruction → emergency.
• Hepatic osteodystrophy and pruritus in chronic cholestasis.

Key differentials of jaundice

• Unconjugated: haemolysis, ineffective erythropoiesis, Gilbert/Crigler-Najjar, resorption of large haematoma, physiological neonatal jaundice.
• Conjugated, hepatocellular: viral hepatitis, alcoholic/drug hepatitis, autoimmune hepatitis, Wilson disease.
• Conjugated, cholestatic/obstructive: choledocholithiasis, carcinoma head of pancreas (painless, progressive jaundice with palpable gallbladder — Courvoisier sign), cholangiocarcinoma, primary biliary cholangitis, primary sclerosing cholangitis, Dubin-Johnson/Rotor.

High-yield: Courvoisier's law — in a jaundiced patient, a palpable, non-tender gallbladder is unlikely to be due to stones (which cause a fibrotic, contracted gallbladder); suspect malignant obstruction (carcinoma head of pancreas/periampullary).

Recently asked / exam angle

• "Bilirubinuria with absent urinary urobilinogen and clay stools" → complete extrahepatic obstruction (single best answer).
• Rate-limiting enzyme of bile acid synthesis → 7α-hydroxylase (CYP7A1).
• Rate-limiting step of bilirubin metabolism → canalicular secretion of conjugated bilirubin (MRP2).
• Enzyme that opens the haem ring releasing CO and iron → haem oxygenase.
• Black liver on biopsy → Dubin-Johnson syndrome (defective MRP2).
• Fasting-induced unconjugated hyperbilirubinaemia → Gilbert syndrome.
• Earliest marker of acute hepatic synthetic failure → prothrombin time/INR (factor VII).
• Clotting factor NOT synthesised by liver → factor VIII (and vWF).
• Antidote for paracetamol → N-acetylcysteine; precipitating toxic metabolite → NAPQI.
• Site of bile acid reabsorption → terminal ileum.
• Basal ganglia nucleus classically damaged in kernicterus → globus pallidus/subthalamic nucleus.

Rapid revision

1. UCB = lipid-soluble, albumin-bound, never in urine; CB = water-soluble, appears in urine (bilirubinuria = conjugated).
2. Haem → (haem oxygenase) biliverdin + CO + Fe → (biliverdin reductase) unconjugated bilirubin.
3. Conjugation enzyme = UGT1A1; rate-limiting step = MRP2 canalicular secretion.
4. Direct van den Bergh = conjugated; indirect (needs methanol) = unconjugated.
5. Haemolysis: ↑ unconjugated, ↑ urine urobilinogen, no bilirubinuria, dark stools (acholuric jaundice).
6. Obstruction: ↑ conjugated, bilirubinuria, absent urobilinogen, pale stools, dark urine, ↑ ALP/GGT.
7. AST:ALT > 2 → alcoholic; ALT > AST → viral hepatitis/NAFLD.
8. PT/INR = earliest synthetic failure marker; factor VIII spared in liver disease.
9. Vitamin K corrects PT in obstructive jaundice but not in hepatocellular failure.
10. Crigler-Najjar II responds to phenobarbitone, type I does not; Gilbert worsens on fasting.
11. Dubin-Johnson = black liver (conjugated); Rotor = normal-coloured liver (conjugated).
12. USG first-line, MRCP best non-invasive, ERCP best when therapy needed; Courvoisier sign → malignant obstruction.