Serotonin Unit Converter
Serotonin Unit Converter
Convert blood serotonin between ng/mL, µg/L and nmol/L — and note first that whole-blood serotonin is almost entirely platelet-bound, so a thrombocytosis raises it with no carcinoid in sight.
Serotonin converter
Mass ⇄ molarWhole-blood serotonin 120 ng/mL, with a platelet count of 250 ×10⁹/L
The three units, and the platelet correction that is not in them
nmol/L = ng/mL × 5.67472
derived from a molecular weight of 176.22 Da for 5-hydroxytryptamine (C₁₀H₁₂N₂O)
- ng/mL = µg/L
- a nanogram per millilitre and a microgram per litre are the same concentration, and one or the other is what almost every report and every published interval uses
- × 5.67472
- the molar factor, from a molecular weight of 176.22 daltons. Serotonin is a small, well-defined molecule, so unlike the peptide hormones on this site it has an exact molar conversion: 100 ng/mL is 567 nmol/L
- the result is a platelet measurement
- over 95% of the serotonin in blood is stored in platelet dense granules, taken up from the portal circulation by the platelet serotonin transporter. Whole-blood serotonin therefore scales with the platelet count, and a thrombocytosis raises it with no change in serotonin synthesis anywhere
- why plasma is worse, not better
- platelet-poor plasma contains only the small free fraction, and any platelet activation during collection or separation releases granule serotonin into it. The result depends on venepuncture technique and centrifugation more than on the patient, which is why Mayo states plainly that platelet-poor plasma specimens are not suitable
- the test that actually answers the question
- for suspected carcinoid syndrome, a 24-hour urinary 5-HIAA collected on a restricted diet is the preferred biochemical test. Blood serotonin is a secondary, confirmatory measurement, and it is not what the diagnosis rests on
Worked example
Whole-blood serotonin 120 ng/mL, with a platelet count of 250 ×10⁹/L
120 ng/mL = 120 µg/L — the same number in the other mass convention
120 × 5.67472 = 681 nmol/L, the molar equivalent
Against the quoted interval, 0–330 ng/mL (0–1,873 nmol/L), 120 ng/mL sits comfortably inside it
Now change one thing that has nothing to do with serotonin. If the same patient developed a reactive thrombocytosis and the platelet count doubled to 500 ×10⁹/L, the whole-blood serotonin would rise towards 240 ng/mL — because the measurement is largely a count of platelet granules, and the granules doubled
The serotonin per platelet has not changed, no tumour has appeared, and the report now reads close to the upper limit. That is why the platelet count belongs beside this result, and why urinary 5-HIAA rather than blood serotonin is the test for carcinoid
Where blood serotonin actually is, and what that does to the result
| Compartment | Share of blood serotonin | What moves the measured value |
|---|---|---|
| Platelet dense granules | More than 95% | The platelet count itself — a thrombocytosis raises whole-blood serotonin proportionately and a thrombocytopenia lowers it |
| Free in plasma | A very small fraction | Platelet activation during venepuncture, delay before separation, centrifugation speed — technique, not physiology |
| Sequestered in the tube after collection | Variable | Uptake continues ex vivo unless the specimen is collected correctly; Mayo’s whole-blood tube contains ascorbic acid as an antioxidant |
What to measure when carcinoid syndrome is the question
| Test | Role | Comment |
|---|---|---|
| 24-hour urinary 5-HIAA | First-line | The serotonin metabolite, and the test of choice for midgut carcinoid. Needs a restricted diet and a complete collection |
| Chromogranin A | First-line, general marker | Reflects neuroendocrine tumour burden rather than serotonin. Raised by proton pump inhibitors, which must be stopped first |
| Blood serotonin | Secondary, confirmatory | Interpret with the platelet count. Metastatic midgut carcinoid typically gives values several times the upper limit |
| Platelet count | Essential context, not a tumour marker | Without it a raised whole-blood serotonin cannot be attributed to anything |
| Plasma or serum 5-HIAA | Emerging alternative | Correlates with the 24-hour urinary excretion and avoids the collection problem, but is less widely available |
A platelet measurement wearing a hormone’s name
The trap on this page comes before the arithmetic. More than ninety-five per cent of the serotonin in blood is not circulating freely at all: it is stored inside platelet dense granules, having been taken up from the portal circulation by the platelet serotonin transporter as blood passes the gut, where enterochromaffin cells make the great majority of the body’s serotonin. A whole-blood serotonin is therefore, to a first approximation, a count of platelet granules. It rises when the platelet count rises and falls when it falls, with no change in serotonin synthesis anywhere in the body. A reactive thrombocytosis — from infection, iron deficiency, inflammation, surgery or a myeloproliferative disorder — will raise a whole-blood serotonin towards or past the upper limit of normal without a carcinoid tumour existing. Read this result with the full blood count in front of you or do not read it at all.
Plasma serotonin fails for the mirror-image reason, and swapping specimen does not rescue the measurement. Platelet-poor plasma contains only the small free fraction, and any platelet activation during venepuncture, transport or centrifugation releases granule serotonin into that plasma and inflates it unpredictably. The result then reflects the technique of the person who took the blood more than the biology of the patient, which is why reference laboratories state that platelet-poor plasma is not a suitable specimen and quote intervals only for whole blood collected into a tube containing ascorbic acid.
The consequence for practice is that serotonin is not the test for carcinoid syndrome. A 24-hour urinary 5-HIAA — the serotonin metabolite, converted on its own page on this site — is the preferred biochemical test for midgut carcinoid, collected on a restricted diet, with chromogranin A as the general neuroendocrine tumour marker beside it. Blood serotonin is a secondary, confirmatory measurement: metastatic midgut carcinoid characteristically produces values several times the upper limit of normal, and measuring all three analytes together gives the best sensitivity when the syndrome is genuinely suspected. What blood serotonin cannot do is establish or exclude the diagnosis on its own.
The units, at least, are exact. Serotonin — 5-hydroxytryptamine — is a small molecule of 176.22 daltons, so the molar conversion is a straightforward one rather than the nominal approximation that peptide and glycoprotein analytes are stuck with. Nanograms per millilitre and micrograms per litre are the same number, and multiplying by 5.67472 gives nanomoles per litre: 100 ng/mL is 567 nmol/L. Diet matters here as it does for 5-HIAA, because serotonin-rich foods and tryptophan supplements raise measured values, and so do several drugs.
Frequently asked questions
Why does a high blood serotonin not mean carcinoid syndrome?
Because whole-blood serotonin is mostly a platelet measurement. Over 95% of it is stored in platelet dense granules, so the result scales with the platelet count: a reactive thrombocytosis raises it towards or beyond the upper limit with no tumour present. Check the platelet count first, and use a 24-hour urinary 5-HIAA if carcinoid is genuinely suspected.
How do I convert serotonin from ng/mL to nmol/L?
Multiply by 5.67472, from a molecular weight of 176.22 daltons for 5-hydroxytryptamine. A serotonin of 100 ng/mL is 567 nmol/L and 120 ng/mL is 681 nmol/L. Nanograms per millilitre and micrograms per litre are the same number.
Should serotonin be measured in whole blood or plasma?
Whole blood, collected into the tube the laboratory specifies — Mayo’s contains ascorbic acid. Platelet-poor plasma holds only the small free fraction and is contaminated by any platelet activation during collection or separation, so reference laboratories state that plasma specimens are not suitable and publish intervals only for whole blood.
What is a normal blood serotonin level?
One named assay quotes 330 ng/mL or less for whole blood (Mayo test SERWB), which is 1,873 nmol/L. Values above about 400 ng/mL warrant investigation and metastatic midgut carcinoid typically exceeds 1,000 ng/mL — but the platelet count must be seen alongside the result before any of those thresholds means anything.
Which test is preferred for carcinoid syndrome?
A 24-hour urinary 5-HIAA, collected on a diet restricted in serotonin-rich foods, with chromogranin A as the general neuroendocrine marker. Blood serotonin is a secondary confirmatory test. Measuring all three together gives the best sensitivity, but 5-HIAA rather than serotonin is what the diagnosis rests on.
Related calculators
References
- Mayo Clinic Laboratories. Test SERWB: Serotonin, Blood — reference value ≤330 ng/mL; whole blood EDTA with ascorbic acid; platelet-poor plasma specimens are not suitable; first-line testing for carcinoid is urinary 5-HIAA and chromogranin A. Accessed 2026.
- Kema IP, de Vries EG, Muskiet FA. Clinical chemistry of serotonin and metabolites. J Chromatogr B Biomed Sci Appl. 2000;747(1–2):33–48.
- Brand T, Anderson GM. The measurement of platelet-poor plasma serotonin: a systematic review of prior reports and recommendations for improved analysis. Clin Chem. 2011;57(10):1376–1386.
- Berger M, Gray JA, Roth BL. The expanded biology of serotonin. Annu Rev Med. 2009;60:355–366.
Medical Disclaimer: The tools and content provided here are for educational and reference purposes only. They are not intended to substitute for professional medical advice, diagnosis, or treatment. Clinical decisions should always be based on the comprehensive assessment of a qualified healthcare professional.