Conditions · Metabolism: amino acids and the urea cycle
Carcinoid syndrome !
A neuroendocrine tumour releases serotonin into the blood, causing flushing, diarrhoea and damage to the right-sided heart valves.
Updated 2026-11-01
Structures involved
Tap one to see what it is and open it in 3D.
- Ileum
- Liver
- Tricuspid anterior leaflet
- Tricuspid septal leaflet
- Tricuspid posterior leaflet
- Pulmonary anterior cusp
- Pulmonary right cusp
- Pulmonary left cusp
- Right ventricle
- Skin
In biochemistry
- Serotonin and melatonin synthesis: Step 2 · Aromatic L-amino acid decarboxylase (AADC)
- Tryptophan
- Molecules derived from amino acids
- Serotonin (5-hydroxytryptamine)
In histology
Why it happens
- Histamine, serotonin, bradykinin and the plasma protein cascades
- Mediators of inflammation: the table to know
- Paraneoplastic syndromes: the table to know
What it is
A set of symptoms from substances (mainly serotonin) released by a well-differentiated neuroendocrine tumour, almost always of the small bowel with liver metastases.
Who gets it
A minority of people with neuroendocrine tumours develop it, almost always middle-aged or older adults.
Causes
- Neuroendocrine tumour of the ileum with liver metastases (most common)
- Bronchial or ovarian tumours, which drain straight into the systemic circulation
Risk factors
- Midgut neuroendocrine tumour
- Liver metastases
Pathophysiology
Tumour cells turn much of the tryptophan into serotonin. While the tumour drains to the portal vein, the liver inactivates serotonin with monoamine oxidase; liver metastases release it into the hepatic veins. Serotonin causes diarrhoea and fibrosis of the tricuspid and pulmonary valves (the lung breaks it down before the left heart). Bradykinins and histamine cause flushing. Spending tryptophan lowers niacin synthesis.
Symptoms and signs
- Sudden flushing of face and neck
- Watery diarrhoea
- Wheezing
- Abdominal pain
- Oedema and signs of right heart failure
Diagnosis
- Clinical
- Flushing with diarrhoea in a patient with a liver mass or bowel tumour.
- Tests
- 5-Hydroxyindoleacetic acid (5-HIAA) in 24-hour urine or plasma; chromogranin A; biopsy with neuroendocrine markers.
- Imaging
- Abdominal CT or MRI; PET with labelled somatostatin analogues; echocardiography for the valves.
Treatment
- Medical
- Somatostatin analogues (octreotide, lanreotide) to control symptoms and growth
- Medical
- Telotristat, which inhibits tryptophan hydroxylase, for diarrhoea that does not respond
- Surgical
- Resection of the primary tumour and liver metastases when possible; valve replacement for advanced heart disease
- Procedure
- Embolisation or ablation of liver metastases
- Acute
- Prevention of carcinoid crisis with somatostatin analogues before surgery
Drugs for this condition
Complications
- Carcinoid heart disease (tricuspid regurgitation and pulmonary stenosis)
- Carcinoid crisis
- Pellagra from niacin deficiency
- Mesenteric fibrosis
Prognosis
The tumours usually grow slowly; carcinoid heart disease is the main cause of death and worsens the outlook.
Test yourself
Think of the answer, then tap the question to see it.
Why does carcinoid syndrome usually appear only with liver metastases?
The liver inactivates serotonin arriving by the portal vein; metastases release it straight into the hepatic veins.
Which valves does it damage, and why those?
Tricuspid and pulmonary, because the lung breaks serotonin down before it reaches the left heart.
Which urinary metabolite is measured for diagnosis?
5-HIAA, the breakdown product of serotonin.
Why can pellagra appear?
The tumour spends tryptophan on serotonin, leaving little to make niacin.
Related conditions
Educational information, not medical advice. For any symptom, see a health professional.
Sources: Carcinoid syndrome: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2026); 5-HIAA urine test: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2026); Serotonin and melatonin biosynthesis (Reactome, 2026).
Spotted a mistake?We are always improving for the community. Tell us what is wrong and our team will review it.Report a mistake
Representative model, for educational purposes only.
