Serotonin (5-Hydroxy Tryptamine)

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Serum serotonin measurement.

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Serotonin (5-Hydroxy Tryptamine) Test Information Guide

Why is it done?
- Measures circulating serotonin levels in blood plasma or serum to assess neurotransmitter status and metabolic function
- Diagnose carcinoid syndrome when elevated urinary 5-hydroxyindoleacetic acid (5-HIAA) is detected, indicating possible carcinoid or neuroendocrine tumors
- Evaluate suspected neuroendocrine tumors, particularly serotonin-secreting carcinoid tumors of the gastrointestinal tract or lungs
- Assess for medullary thyroid carcinoma and other serotonin-producing malignancies
- Investigate symptoms of carcinoid syndrome including flushing, diarrhea, wheezing, and heart palpitations
- Monitor treatment response in patients with known neuroendocrine tumors or carcinoid disease
- Typically performed when clinical symptoms suggest carcinoid syndrome or when imaging studies reveal suspicious neuroendocrine lesions
Normal Range
- Standard Reference Range: 50-200 ng/mL (or 0.2-1.0 µmol/L), with specific ranges varying by laboratory method
- Units of Measurement: Nanograms per milliliter (ng/mL) or micromoles per liter (µmol/L)
- Normal Result (Negative): Serotonin levels within laboratory reference range indicate normal serotonin metabolism and make carcinoid syndrome or active neuroendocrine tumors less likely
- Elevated Result (Positive/Abnormal): Serotonin >200 ng/mL suggests possible neuroendocrine tumor or carcinoid syndrome, particularly if accompanied by elevated 5-HIAA in 24-hour urine collection
- Borderline Values: Results in the 180-220 ng/mL range may warrant repeat testing or additional investigations to confirm findings
- Important Note: Approximately 5% of healthy individuals may have slightly elevated serotonin levels; clinical interpretation must integrate symptomatology and other test results
Interpretation
- Markedly Elevated Levels (>300 ng/mL): Strongly suggests active carcinoid syndrome or serotonin-secreting neuroendocrine tumor; recommend additional imaging studies (CT/MRI) and chromogranin A testing
- Moderately Elevated Levels (200-300 ng/mL): Suggestive of possible neuroendocrine tumor; correlate with 24-hour urine 5-HIAA levels and clinical symptoms for diagnostic confirmation
- Mildly Elevated Levels (150-200 ng/mL): May indicate early-stage disease or benign elevation; requires clinical correlation and possibly repeat testing to establish trend
- Normal Range (50-200 ng/mL): Reduces probability of active carcinoid syndrome; if symptoms persist, investigate alternative diagnoses or repeat testing after appropriate preparation
- Low/Undetectable Levels (<50 ng/mL): Effectively excludes carcinoid syndrome; may suggest alternative etiologies for presenting symptoms
- Factors Affecting Interpretation: Diet (foods high in serotonin: bananas, nuts, chocolate), medications (SSRIs, TCAs, MAO inhibitors), stress levels, platelet aggregation, hemolysis during collection, and timing of blood draw affect results
- Clinical Correlation Essential: Serotonin alone is insufficient for diagnosis; must be combined with symptoms, imaging, chromogranin A levels, and 5-HIAA to make definitive clinical diagnosis
Associated Organs
- Primary Organ Systems:
- Gastrointestinal tract (small intestine, colon) - primary site for carcinoid tumors
- Neuroendocrine system - enterochromaffin cells produce serotonin
- Lungs - bronchial carcinoid tumors may secrete serotonin
- Pancreas - pancreatic neuroendocrine tumors may secrete serotonin
- Thyroid - medullary thyroid carcinoma may produce serotonin
- Central nervous system - regulates serotonin synthesis and metabolism
- Cardiovascular system - affected by excessive serotonin leading to carcinoid heart disease
- Diseases and Conditions Associated with Abnormal Results:
- Carcinoid syndrome - complex of symptoms caused by excessive serotonin from neuroendocrine tumors
- Carcinoid tumors - neuroendocrine neoplasms originating from enterochromaffin cells
- Neuroendocrine tumors (NETs) - various malignancies producing serotonin
- Medullary thyroid carcinoma - serotonin-producing thyroid malignancy
- Carcinoid heart disease - valvular fibrosis from chronic serotonin exposure
- Foregut carcinoids - serotonin-secreting tumors from stomach, pancreas, or lungs
- Midgut carcinoids - most common type of serotonin-secreting carcinoid tumors
- Complications Associated with Abnormal Results:
- Carcinoid heart disease with tricuspid and pulmonary valve involvement
- Acute carcinoid crisis - severe hypertension and bronchospasm during tumor manipulation
- Mesenteric fibrosis - ischemic complications from tumor metastases
- Pulmonary hypertension - from chronic serotonin exposure
- Malabsorption and nutritional deficiencies - tryptophan diverted to serotonin synthesis
- Bowel obstruction - from metastatic disease or fibrosis
Follow-up Tests
- Recommended Follow-up Tests if Elevated:
- 24-hour urine 5-hydroxyindoleacetic acid (5-HIAA) - gold standard for carcinoid syndrome diagnosis; should exceed 8 mg/24 hours to confirm
- Plasma chromogranin A (CgA) - general neuroendocrine tumor marker; typically elevated in carcinoid disease
- Chromogranin B and pancreastatin - alternative neuroendocrine markers for tumor burden assessment
- Neurokinin A levels - often elevated in carcinoid syndrome, particularly midgut tumors
- CT or MRI imaging - to localize primary tumor and identify metastatic disease
- Somatostatin receptor scintigraphy (octreoscan) - nuclear imaging to detect somatostatin receptor-positive neuroendocrine tumors
- PET imaging with 68Ga-DOTATATE - superior sensitivity for detecting neuroendocrine tumors and metastases
- Upper and lower endoscopy - direct visualization to identify source of tumor
- Electrocardiography and echocardiography - evaluate for carcinoid heart disease and valvular changes
- Repeat Testing Frequency:
- Initial diagnosis - repeat serotonin test after patient dietary and medication preparation to confirm results
- Confirmed disease - monitor every 3-6 months or per oncology recommendation
- Post-treatment - serially measure to assess therapeutic response and detect recurrence
- Stable disease - may retest annually or as clinically indicated
Fasting Required?
- Fasting Status: NO - Fasting is not typically required for serotonin testing
- Special Dietary Preparation (Strongly Recommended): Follow serotonin-restricted diet for 3-5 days prior to blood collection
- Foods to Avoid Before Testing:
- Bananas and plantains - extremely high in serotonin and 5-HT metabolites
- Pineapple and kiwi - significant serotonin content
- Nuts and seeds - walnuts, almonds, cashews, sunflower seeds
- Chocolate and cocoa products - moderate serotonin levels
- Tomatoes and tomato products - moderate serotonin and tryptophan
- Avocados - moderate serotonin content
- Plums, prunes, and plantains
- Medications to Discontinue or Avoid (3-5 days before test if possible):
- Selective serotonin reuptake inhibitors (SSRIs) - sertraline, paroxetine, fluoxetine, citalopram
- Tricyclic antidepressants (TCAs) - amitriptyline, nortriptyline, imipramine
- Monoamine oxidase inhibitors (MAOIs) - phenelzine, tranylcypromine, isocarboxazid
- Amphetamines and sympathomimetics - elevate serotonin levels
- Lithium - affects serotonin metabolism
- Levodopa and bromocriptine - can affect serotonin levels
- Other Patient Preparation Requirements:
- Rest and stress reduction - avoid strenuous exercise for 24 hours prior to test; mental stress affects serotonin levels
- Morning blood draw preferred - collect sample between 8-10 AM when possible
- Avoid excessive alcohol consumption - can interfere with serotonin metabolism
- Special sample handling - blood must be collected in appropriate tube, kept cool, and processed promptly to prevent false elevation from platelet breakdown
- Avoid tryptophan supplementation - discontinue at least one week before testing
- Consult with laboratory and physician - specific preparation protocols may vary by institution and clinical context

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