Carcinoid Syndrome
Gastroenterology
Illness script · Gastroenterology
Carcinoid Syndrome
Paraneoplastic syndrome from serotonin-secreting well-differentiated neuroendocrine tumors (NETs) that have metastasized to the liver, bypassing hepatic degradation.
This illness script for Carcinoid Syndrome covers predisposing factors, classic presentation, mechanism, workup, management, and the clinical pivots that separate it from look-alikes—written for USMLE Step 1 and clerkship reasoning.
01
Predisposing factors
- Peak incidence 5th–6th decade; slight female predominance
- Most common primary site: ileum (most likely to metastasize); appendix (most common primary but rarely metastasizes)
- Syndrome requires hepatic metastases OR primary draining to systemic circulation (bronchial, ovarian carcinoids are exceptions)
- MEN-1 association (pancreatic/foregut NETs)
- Only ~10% of all carcinoid tumors produce the syndrome
02
Presentation
- Classic triad: episodic cutaneous flushing (face/neck), secretory diarrhea, wheezing/bronchospasm
- Right-sided valvular disease: tricuspid regurgitation + pulmonic stenosis ("TIPS")
- Mesenteric fibrosis → abdominal pain, bowel obstruction
- Pellagra-like symptoms (niacin deficiency): dermatitis, diarrhea, dementia — tryptophan diverted to serotonin synthesis
- Carcinoid crisis: severe flushing, refractory hypotension, bronchospasm — classic perioperative emergency
03
Pathophysiology
- NETs secrete serotonin, histamine, bradykinin, prostaglandins into systemic circulation
- Liver metastases bypass hepatic first-pass degradation → mediators reach systemic circulation
- Serotonin causes diarrhea + fibrosis (cardiac valves, mesentery)
- Right-sided cardiac lesions spared because lungs degrade serotonin before it reaches left heart
04
Diagnostics
- First-line: 24-hour urine 5-HIAA (serotonin metabolite) — sensitivity ~75%
- Serum chromogranin A: sensitive NET marker but nonspecific
- Pitfall: false-positive 5-HIAA with bananas, walnuts, avocados, pineapple, or SSRIs — restrict diet before collection
- Best imaging: Ga-68 DOTATATE PET (preferred) or Octreoscan (somatostatin receptor scintigraphy)
- CT/MRI abdomen to characterize liver metastases and primary tumor
05
Management
- Somatostatin analogs (octreotide, lanreotide) — first-line for symptom control AND anti-tumor effect
- Telotristat (tryptophan hydroxylase inhibitor) for refractory diarrhea despite octreotide
- Surgical resection of primary ± hepatic debulking for cytoreduction
- Carcinoid crisis: IV octreotide bolus; preoperative octreotide prophylaxis is mandatory
- Avoid triggers: alcohol, emotional stress, catecholamine-releasing drugs, anesthesia without prophylaxis
- Niacin supplementation to prevent pellagra
06
Clinical pivots
How to separate this script from the look-alikes that show up on exams and on the wards.
Pheochromocytoma
Pheo produces hypertensive crises during flushing; carcinoid flushing is normotensive or hypotensive, with elevated 5-HIAA not catecholamines.
VIPoma (Verner-Morrison syndrome)
VIPoma causes massive watery diarrhea + hypokalemia + achlorhydria (WDHA) with NO flushing or wheezing; elevated VIP not serotonin.
Mastocytosis
Mastocytosis also causes flushing and diarrhea but is identified by urticaria pigmentosa, elevated serum tryptase, and skin biopsy — no 5-HIAA elevation.
Irritable Bowel Syndrome
IBS has no episodic flushing, no wheezing, no right-sided cardiac lesions, and a normal 24-hour urine 5-HIAA.
Keep reading
Full library- BronchiolitisViral lower respiratory tract infection causing small airway inflammation and obstruction, predominantly in infants <2 years old.
- CellulitisAcute bacterial infection of the deep dermis and subcutaneous fat, most often caused by Streptococcus pyogenes or Staphylococcus aureus.
Educational use only. This illness script is a study framework, not medical advice. Confirm decisions with current guidelines and your clinical supervisors.