Neuroendocrine Tumors and Diarrhea

Diarrhea occurs in 50% of patients with carcinoid syndrome, typically from midgut NETs that have metastasized to the liver, allowing vasoactive compounds like serotonin and tachykinins to bypass hepatic metabolism and enter systemic circulation 1, 2
The diarrhea in carcinoid syndrome is specifically described as occurring with intermittent abdominal pain (40% of cases), suggesting an episodic rather than purely continuous pattern 1, 3
Carcinoid syndrome can also occur without liver metastases if there is direct retroperitoneal involvement with venous drainage bypassing the liver 1
Gastrinomas (Zollinger-Ellison syndrome) cause diarrhea in 50-65% of patients through gastric acid hypersecretion, though symptoms are typically chronic and persistent rather than intermittent 5
VIPomas produce profuse secretory diarrhea (>1 liter/day) in 100% of cases with severe dehydration and hypokalemia—this is typically continuous and life-threatening rather than intermittent 5
Glucagonomas cause diarrhea along with necrolytic migratory erythema, weight loss, and diabetes mellitus 1
Somatostatinomas cause diarrhea and steatorrhea along with cholelithiasis and diabetes mellitus 1

Clinical Context and Differential Diagnosis

Most gastroenteropancreatic NETs (the majority) are non-functioning and present with mass effects rather than hormonal symptoms 1, 3
Carcinoid syndrome occurs in only ~20% of well-differentiated midgut NETs, and is very rare with rectal NETs 1, 2

Measure 24-hour urinary 5-HIAA (88% sensitivity and specificity for carcinoid syndrome) with dietary restrictions 4
Check fasting serum gastrin if acid reflux and diarrhea coexist; levels >1000 pg/mL suggest gastrinoma 4
Measure serum VIP during a diarrheal episode if secretory diarrhea exceeds 1 liter/day 4
Plasma chromogranin A serves as a general NET marker, particularly useful in gastric carcinoids with metastases 4