Unit Converter
Serotonin
(5-Hydroxytryptamine, 5-HT - Neurotransmitter & Neuroendocrine Biomarker, Key for Carcinoid Syndrome Evaluation)
Synonyms
- Serotonin
- 5-Hydroxytryptamine (5-HT)
- Enterochromaffin hormone
- Peripheral serotonin
- Platelet serotonin
- Serum/Plasma serotonin
Units of Measurement
- µmol/L
- µg/L
- µg/dL
- µg/100 mL
- µg%
- ng/mL
Unit Conversions
Molecular weight of Serotonin = 176.22 g/mol
µmol/L ↔ µg/L
1 µmol/L=176.22 µg/L1\ \text{µmol/L} = 176.22\ \text{µg/L}1 µmol/L=176.22 µg/L 1 µg/L=0.00567 µmol/L1\ \text{µg/L} = 0.00567\ \text{µmol/L}1 µg/L=0.00567 µmol/L
ng/mL ↔ µg/L
1 ng/mL=1 µg/L1\ \text{ng/mL} = 1\ \text{µg/L}1 ng/mL=1 µg/L
µg/dL → µg/L
1 µg/dL=10 µg/L1\ \text{µg/dL} = 10\ \text{µg/L}1 µg/dL=10 µg/L
µg% = µg/dL
Description
Serotonin (5-HT) is a neurotransmitter and vasoactive amine synthesized mainly in:
- Enterochromaffin cells of the gastrointestinal tract (≈90%)
- Central nervous system neurons
- Platelets (storage and release)
It regulates:
- Mood and behavior
- Gut motility
- Vasoconstriction
- Platelet aggregation
In laboratory medicine, plasma or serum serotonin is primarily used to evaluate carcinoid syndrome and neuroendocrine tumors (NETs).
Physiological Role
Central Nervous System
- Mood regulation
- Appetite
- Sleep–wake cycle
- Pain modulation
Gastrointestinal System
- Stimulates intestinal motility
- Involved in nausea/vomiting pathways
Cardiovascular
- Vasoconstriction / vasodilation
- Platelet aggregation (stored in platelets)
Endocrine & Metabolic
- Influences insulin secretion
- Interacts with gut–brain axis
Clinical Significance
HIGH SEROTONIN
Most important for diagnosing carcinoid syndrome.
1. Carcinoid Tumors / Neuroendocrine Tumors (NETs)
Especially midgut NETs that overproduce serotonin.
Symptoms:
- Flushing
- Diarrhea
- Wheezing
- Right-sided heart disease
- Abdominal pain
Serotonin can be very high in advanced disease.
2. Serotonin Syndrome
Due to serotonergic drug toxicity:
- SSRIs, MAOIs, tramadol, MDMA
Serum serotonin is NOT diagnostic; diagnosis is clinical.
3. Intestinal Obstruction or Inflammation
4. Postprandial Rise
High tryptophan or banana/pineapple intake.
5. Medications
- Lithium
- SSRIs/SNRIs
(may slightly affect levels)
LOW SEROTONIN
May occur in:
- Depression (not routinely measured)
- Platelet dysfunction
- Chronic illness/malnutrition
- Vitamin B6 deficiency (cofactor for serotonin synthesis)
Clinical interpretation is limited.
Reference Intervals
(Tietz 8E + Mayo + ARUP + Endocrine Society NET Guidelines)
Plasma / Serum Serotonin
- 50 – 220 ng/mL
(= 50 – 220 µg/L)
(= 0.28 – 1.25 µmol/L)
Carcinoid Syndrome Indicators
- > 400 ng/mL → suggestive
- > 1000 ng/mL → strongly indicative of serotonin-secreting NET
- > 2000 ng/mL → advanced metastatic disease
Urinary 5-HIAA (Preferred for diagnosis)
Serotonin → metabolized → 5-HIAA
- Normal: <6 mg/day
- >25 mg/day = diagnostic for carcinoid syndrome
Diagnostic Uses
1. Carcinoid Syndrome / NETs (Primary Use)
- Elevated plasma serotonin suggests active tumor secretion
- Evaluate with:
- 5-HIAA (gold standard)
- Chromogranin A
- CT/MRI, DOTATATE PET
- 5-HIAA (gold standard)
2. Diarrheal Episode Investigation
High serotonin may explain secretory diarrhea.
3. Serotonin-Producing Tumors Outside GI Tract
e.g., bronchial NETs.
4. Monitoring Treatment
Levels correlate with:
- Tumor burden
- Symptom severity
- Response to therapy (somatostatin analogs)
Analytical Notes
- Fasting sample preferred
- Avoid serotonin-rich foods 24 hours prior:
- Banana
- Pineapple
- Kiwi
- Avocado
- Tomato
- Plum
- Banana
- EDTA or heparin plasma
- Process rapidly (platelets release serotonin)
- HPLC, LC–MS/MS, or immunoassay
Clinical Pearls
- Plasma serotonin is not the primary screening tool - urinary 5-HIAA is superior.
- Severe carcinoid syndrome can cause right-sided valvular heart disease.
- Elevated serotonin + high 5-HIAA strongly supports NET activity.
- SSRIs increase CNS serotonin but often do not significantly raise plasma serotonin.
- Serotonin is stored in platelets → platelet-rich samples give falsely high results.
Interesting Fact
Serotonin was first isolated in 1948 from serum (“sero-”) and recognized for its ability to increase vascular tone (“-tonin”).
References
- Tietz Clinical Chemistry & Molecular Diagnostics, 8th Edition - Biogenic Amines
- Endocrine Society Guidelines - Neuroendocrine Tumors
- Mayo Clinic Laboratories - Serotonin
- ARUP Consult - Serotonin & 5-HIAA
- NIH / MedlinePlus - Serotonin Test
- World Journal of Gastroenterology - Carcinoid Syndrome Review