5-HT

Serotonin

Hormones

What is Serotonin?

Serotonin (5-hydroxytryptamine, 5-HT) is a monoamine neurotransmitter and hormone synthesized from the amino acid tryptophan. Contrary to popular belief, approximately 90–95% of the body's serotonin is produced in enterochromaffin cells of the gastrointestinal tract, not the brain. Gut serotonin regulates intestinal motility, secretion, and sensation. The remaining 5–10% is synthesized in serotonergic neurons of the brainstem raphe nuclei, where it modulates mood, anxiety, sleep, appetite, cognition, and pain perception. Platelets do not produce serotonin but actively take it up from plasma and store it in dense granules, releasing it upon activation to promote vasoconstriction and platelet aggregation.

Clinically, blood serotonin measurement is used primarily to detect carcinoid tumors (neuroendocrine tumors that overproduce serotonin). Whole blood serotonin reflects platelet serotonin stores and is the standard measurement. The serotonin metabolite 5-hydroxyindoleacetic acid (5-HIAA), measured in 24-hour urine, is the preferred screening test for carcinoid syndrome because it is more specific and less affected by diet.

Why It Matters

Serotonin measurement is clinically important primarily for detecting carcinoid tumors and monitoring treatment response in carcinoid syndrome. Carcinoid tumors—now classified as well-differentiated neuroendocrine tumors—are often slow-growing but can metastasize, particularly to the liver, where they produce carcinoid syndrome (flushing, diarrhea, wheezing, and right-sided heart valve disease). Early detection through serotonin/5-HIAA measurement allows timely treatment. Serotonin is not used to diagnose psychiatric conditions like depression despite its role in mood regulation, because peripheral levels do not reflect brain serotonin.

Normal Reference Ranges

GroupRangeUnit
Whole Blood Serotonin≤330ng/mL

Reference ranges may vary by laboratory. Always compare results to the ranges provided by your testing facility.

What High 5-HT Levels Mean

Common Causes

  • Carcinoid tumors (neuroendocrine tumors of the GI tract, lung, or other sites)
  • Carcinoid syndrome (metastatic carcinoid, especially with liver metastases)
  • Medications: SSRIs, MAO inhibitors, tryptophan supplements
  • Medullary thyroid carcinoma (occasionally)
  • Small bowel obstruction

Possible Symptoms

  • Episodic flushing (face and upper body)
  • Profuse watery diarrhea
  • Bronchospasm and wheezing
  • Right-sided heart valve disease (carcinoid heart disease)
  • Abdominal pain and cramping
  • Telangiectasias (spider-like blood vessels)
  • Pellagra-like symptoms (niacin deficiency from tryptophan diversion)

What to do: Elevated whole blood serotonin or urinary 5-HIAA should prompt evaluation for carcinoid tumor. Medication interferences should be reviewed, but dietary restrictions are mainly relevant to urinary 5-HIAA rather than whole blood serotonin. Confirmatory testing includes 24-hour urine 5-HIAA with dietary restrictions for 3 days before collection. Chromogranin A is a complementary tumor marker. Imaging includes CT/MRI for tumor localization and somatostatin receptor scintigraphy (OctreoScan) or 68Ga-DOTATATE PET/CT. Treatment includes surgical resection when possible, somatostatin analogs (octreotide, lanreotide) for symptom control and tumor stabilization, and echocardiography to assess for carcinoid heart disease.

What Low 5-HT Levels Mean

Common Causes

  • Carcinoid tumor after successful treatment
  • Mastocytosis treatment
  • Certain medications (reserpine)
  • Phenylketonuria (PKU)
  • Tryptophan-deficient diet
  • Down syndrome (reduced platelet serotonin)

Possible Symptoms

  • Low peripheral serotonin is rarely symptomatic
  • Central serotonin depletion (not measurable peripherally) is associated with depression, anxiety, and sleep disturbance
  • Reduced platelet aggregation (theoretical)

What to do: Low peripheral serotonin is uncommon and usually not clinically significant on its own. If measured after carcinoid tumor treatment, low or normalizing serotonin/5-HIAA indicates treatment response. Low serotonin should not be used to diagnose depression or anxiety—peripheral serotonin does not reflect brain serotonin. If mood or psychiatric symptoms are present, evaluation by a mental health professional is appropriate, and treatment decisions are based on clinical assessment, not blood serotonin levels.

When Is 5-HT Testing Recommended?

  • When carcinoid syndrome is suspected (flushing, diarrhea, wheezing)
  • When a neuroendocrine tumor is identified on imaging or biopsy
  • When monitoring treatment response in known carcinoid tumor
  • When evaluating unexplained right-sided heart valve disease

Frequently Asked Questions

No. Despite serotonin's well-known role in mood regulation, a blood serotonin test cannot diagnose depression or any psychiatric condition. The serotonin measured in blood comes primarily from gut enterochromaffin cells and is stored in platelets—it does not cross the blood-brain barrier and has no relationship to serotonin levels in the brain's synapses. Depression is a complex disorder diagnosed through clinical evaluation, not blood tests. The effectiveness of SSRIs (selective serotonin reuptake inhibitors) in treating depression does not prove that depression is caused by low serotonin—the mechanism is more complex. Blood serotonin testing is indicated only for detecting carcinoid tumors.
Dietary restrictions are mainly important for urinary 5-HIAA testing, not whole blood serotonin. Before a 24-hour urine 5-HIAA collection, patients are commonly asked to avoid serotonin-rich foods such as bananas, pineapple, kiwi, plums, avocados, walnuts, tomatoes, and eggplant for 3 days. Whole blood serotonin is affected more by specimen handling and certain medications than by diet. Medications to discuss with your doctor include MAO inhibitors, lithium, methyldopa, morphine, reserpine, SSRIs, and supplements containing tryptophan or 5-HTP.
Carcinoid syndrome occurs when a neuroendocrine tumor, typically with liver metastases, secretes vasoactive substances (serotonin, histamine, prostaglandins, tachykinins) directly into the systemic circulation. The liver normally metabolizes serotonin from gut carcinoids before it reaches systemic circulation, which is why carcinoid syndrome typically appears only with liver metastases that bypass hepatic metabolism. Serotonin causes fibrosis of the right-sided heart valves (tricuspid and pulmonary), leading to regurgitation and eventually right heart failure—this is carcinoid heart disease, which occurs in 20–50% of patients with carcinoid syndrome. The left-sided valves are spared because serotonin is inactivated during passage through the lungs. Regular echocardiographic screening is essential in patients with carcinoid syndrome.

Related Biomarkers

DADopamineMTMelatoninCortisolCortisol

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Medical Disclaimer: This information is for educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Reference ranges may vary between laboratories. Always consult your healthcare provider for interpretation of your specific test results.

Disclaimer: SymptomGPT is not a medical diagnosis tool and does not provide medical advice. Always consult a qualified healthcare professional. If you are experiencing a medical emergency, call 911 immediately.