Carcinoid Syndrome & Functioning NETs
Hormone syndromes, carcinoid heart disease, crisis, and their imaging/therapy links
Quick Answer
Functioning neuroendocrine tumors secrete bioactive hormones that produce recognizable clinical syndromes. Carcinoid syndrome (flushing, diarrhea, bronchospasm) is driven mainly by serotonin and usually requires hepatic metastases (or retroperitoneal disease) to bypass first-pass liver clearance; chronic exposure causes right-sided carcinoid heart disease (Hedinger syndrome). Pancreatic functioning tumors cause distinct syndromes (insulinoma, gastrinoma/ZES, glucagonoma, VIPoma, somatostatinoma). All connect to nuclear medicine through SSTR imaging/therapy, and all share the risk of carcinoid crisis — prevented with somatostatin-analog cover around procedures and PRRT.
Carcinoid Syndrome
Mediators (serotonin, histamine, kinins, prostaglandins) released into the systemic circulation produce episodic flushing, secretory diarrhea, and bronchospasm. Because the liver clears these mediators via the portal circulation, the full syndrome usually appears only with hepatic metastases (venous drainage bypassing the liver) or with retroperitoneal/ovarian/bronchial primaries that drain systemically. Urinary 5-HIAA (a serotonin metabolite) is the biochemical marker (diet-sensitive).
Carcinoid Heart Disease (Hedinger Syndrome)
Chronic serotonin exposure causes fibrous plaque deposition on right-sided endocardium and valves — tricuspid regurgitation and pulmonary stenosis/regurgitation — leading to right heart failure. Left-sided disease is spared unless there is a right-to-left shunt or a bronchial primary (mediators reach the left heart before pulmonary clearance). It is a leading cause of morbidity/mortality in carcinoid syndrome; NT-proBNP and echocardiography screen for it.
Functioning Pancreatic NET Syndromes
| Tumor | Hormone | Syndrome |
|---|---|---|
| Insulinoma | Insulin | Fasting hypoglycemia (Whipple triad); usually benign/small |
| Gastrinoma | Gastrin | Zollinger–Ellison — refractory ulcers, diarrhea (MEN1 association) |
| Glucagonoma | Glucagon | Diabetes, weight loss, necrolytic migratory erythema |
| VIPoma | VIP | Watery diarrhea, hypokalemia, achlorhydria (WDHA / Verner–Morrison) |
| Somatostatinoma | Somatostatin | Diabetes, steatorrhea, gallstones |
Insulinomas are often small and SSTR-variable (localization may need EUS/selective sampling); most other functioning PanNETs are SSTR-avid and DOTATATE-visible.
Carcinoid Crisis
A life-threatening surge of vasoactive mediators — profound flushing, hemodynamic instability (hypo- or hypertension), bronchospasm — that can be precipitated by anesthesia, tumor manipulation/biopsy, embolization, and PRRT. Prevention and treatment center on somatostatin analogs (octreotide); it is a key reason NET patients receive octreotide cover around procedures and radionuclide therapy.
Imaging & Biochemical Work-up
- SSTR-PET (DOTATATE) localizes and stages the tumor and gates PRRT (see the NET page); functioning tumors are usually well-differentiated and SSTR-avid.
- Urinary 5-HIAA and chromogranin A (general marker); hormone-specific assays for pancreatic syndromes.
- Echocardiography / NT-proBNP to screen for carcinoid heart disease.
- Insulinoma may require EUS or selective arterial calcium-stimulation when SSTR imaging is negative.
Therapy Links
- Somatostatin analogs (octreotide/lanreotide): symptom control and antiproliferative effect — the backbone (and crisis prophylaxis).
- Telotristat (tryptophan-hydroxylase inhibitor) for refractory carcinoid diarrhea.
- PRRT (¹⁷⁷Lu-DOTATATE) for progressive SSTR-avid disease (with octreotide cover to prevent crisis).
- Liver-directed therapy for hepatic-dominant disease driving the syndrome; valve surgery for carcinoid heart disease.
Common Pitfalls
- Expecting carcinoid syndrome without hepatic (or systemic-draining) disease — a GI primary draining to the portal system usually will not cause the full syndrome.
- Missing carcinoid heart disease (right-sided) in a symptomatic carcinoid patient.
- Precipitating carcinoid crisis at procedures/PRRT without octreotide cover.
- Relying on SSTR imaging for insulinoma, which is often small and SSTR-variable.
Board Pearls
Carcinoid syndrome (flushing, diarrhea, bronchospasm) is mainly serotonin-driven and usually requires hepatic metastases (or systemic-draining disease) to bypass hepatic clearance; chronic exposure causes right-sided carcinoid heart disease (Hedinger syndrome) — tricuspid regurgitation and pulmonary stenosis.
Carcinoid crisis (mediator surge with hemodynamic instability/bronchospasm) can be precipitated by anesthesia, tumor manipulation, embolization, and PRRT — prevented and treated with somatostatin analogs (octreotide cover).
Functioning pancreatic NETs each have a signature: insulinoma (hypoglycemia; often small/SSTR-variable — may need EUS), gastrinoma (ZES ulcers; MEN1), glucagonoma (necrolytic migratory erythema), VIPoma (WDHA), somatostatinoma (diabetes/gallstones/steatorrhea). Most are SSTR-avid (DOTATATE-visible, PRRT-eligible); telotristat targets refractory carcinoid diarrhea, and 5-HIAA is the serotonin marker.
Related Pages
- Disease: Neuroendocrine tumors (grading, imaging, PRRT selection).
- Therapy: ¹⁷⁷Lu-DOTATATE (PRRT); tracer: DOTATATE PET agents.
- Related: Adrenal & pheochromocytoma (catecholamine syndromes).
Figure / Diagram Suggestions
- A carcinoid syndrome schematic (mediator release, hepatic bypass requirement, target organs).
- Right-sided carcinoid heart disease valve diagram (tricuspid/pulmonary).
- A functioning PanNET syndromes table-figure (hormone → syndrome → SSTR avidity).
Self-Check (Board-Style)
Q1. Why does a small-bowel carcinoid usually not cause carcinoid syndrome until it metastasizes to the liver?
Answer: Mediators (serotonin, etc.) drain via the portal vein and are cleared by the liver; only when hepatic metastases drain into the systemic circulation (bypassing first-pass clearance) does the full syndrome appear.
Q2. Which valves does carcinoid heart disease characteristically affect, and why is the left heart usually spared?
Answer: Right-sided — tricuspid (regurgitation) and pulmonary (stenosis/regurgitation). The lungs clear the mediators, sparing the left heart unless there is a right-to-left shunt or a bronchial primary.
Q3. Before PRRT or tumor manipulation in a carcinoid patient, what prophylaxis prevents carcinoid crisis?
Answer: Somatostatin-analog (octreotide) cover — it prevents and treats the mediator surge of carcinoid crisis.
Q4. A functioning pancreatic tumor causes fasting hypoglycemia but is not seen on DOTATATE-PET. Diagnosis and next step?
Answer: Insulinoma — often small and SSTR-variable, so it can be DOTATATE-negative; localize with EUS or selective arterial calcium-stimulation sampling.
Evidence & sources
https://nuc.med/carcinoid-syndrome-functioning-net/ Report an issue