Carcinoids are small, slow-growing neuroendocrine tumors. They are most commonly located in the gastrointestinal tract and can synthesize a variety of hormones (especially serotonin). Most carcinoids are asymptomatic because most of the hormones they produce are metabolized by the first-pass effect in the liver. Carcinoid syndrome – characterized by diarrhea, flushing, dyspnea, and wheezing – may occur if a serotonin-producing tumor has metastasized to the liver, bypassing first-pass metabolism. Elevated 5-HIAA in the urine helps to establish the diagnosis. Imaging techniques such as CT, MRI, and somatostatin receptor scintigraphy are used to detect the primary tumor as well as its metastases. Surgical resection is the first-line treatment. In inoperable cases, somatostatin analogs can be given.
- Carcinoid tumors are neuroendocrine tumors that arise from amine precursor uptake and decarboxylation cells (APUD cells).
- Neuroendocrine cells (including β cells in the pancreas or enterochromaffin cells in the GI tract) belong to the APUD series and share a common biologic function despite their different embryologic origins, anatomic sites, and secretory products (e.g., serotonin, histamine).
Tumor location 
- GI tract (55% of cases): carcinoid
- Pancreas (15% of cases)
- Bronchopulmonary system (10% of cases)
- Thyroid: medullary carcinoma
- Adrenal glands: pheochromocytoma
Secretory products 
- Carcinoid tumors can synthesize different hormones (most commonly serotonin).
- Serotonin is degraded via the following mechanisms:
Serotonin can reach systemic circulation under the following conditions:
- Intestinal carcinoid tumors with liver metastases
- Extraintestinal carcinoid tumors
- ↑ Serotonin in systemic circulation can lead to:
The Measure of thirds for carcinoid tumors: ⅓ are Multiple, ⅓ are associated with another Malignancy, ⅓ Metastasize.
- Diarrhea and abdominal cramps
- Cutaneous flushing
- Dyspnea, wheezing (asthma-like attacks)
- Possible weight loss despite normal appetite
- Carcinoid heart disease 
- Other symptoms: abdominal pain
Subtypes and variants
Carcinoid lung tumor
- Etiology: association with smoking is not as strong as with lung carcinoma
- Clinical features
- Prognosis: excellent (metastasis is rare)
Ovarian strumal carcinoid 
- Clinical biochemistry should be performed first if a carcinoid tumor is suspected.
- Imaging tests are then used to localize and stage the tumor.
- ↑ 5-hydroxyindoleacetic acid (5-HIAA) in 24-hour urine collection
- Fasting plasma 5-HIAA concentration
- Analysis of secretory products (see “ .”)
- ↑ Serotonin serum levels
- CT scans of the abdomen and pelvis
- Somatostatin-receptor scintigraphy
- Abdominal ultrasound: although not part of the workup, sometimes detects a mass if routinely conducted
- Histology: prominent rosettes composed of numerous small monomorphic cells with salt-and-pepper chromatin
- Immunohistochemistry: immunostaining with synaptophysin, chromogranin A, and neuron-specific enolase (NSE) to confirm neuroendocrine origin
|Differential diagnosis of carcinoid tumor|
|Blood glucose||Diarrhea||Associated conditions||Other findings|
|Carcinoid tumor|| || |
|Glucagonoma|| || |
|Insulinoma|| || |
|Gastrinoma|| || |
|VIPoma|| || |
|Somatostatinoma|| || |
|Pheochromocytoma|| || |
Other differential diagnoses
The differential diagnoses listed here are not exhaustive.
Somatostatin analogs (e.g., octreotide) to relieve symptoms
- ↓ 5-HIAA levels in the urine
- High-dose treatment slows progression and increases survival time
- inhibitors (e.g., telotristat) as adjunctive therapy if diarrhea is not controlled with somatostatin analogs
- Palliative chemotherapy: to reduce rapid tumor progression
- Somatostatin analogs (e.g., octreotide) to relieve symptoms