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[Biochemistry](https://durgical.com/en/biochemistry/) · Molecule

# Serotonin (5-hydroxytryptamine)

Serotonin is made from tryptophan. It is a brain neurotransmitter for mood and sleep, and in the gut it controls motility.

Updated 2026-11-01

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## Key facts

- Tryptophan hydroxylase (BH4) and aromatic L-amino acid decarboxylase (B6) make it.
- About 90% is in enterochromaffin cells of the gut; platelets take it up and carry it.
- MAO-A breaks it down to 5-HIAA, which is excreted in urine.
- In the pineal gland it is the precursor of melatonin.

**Also called**
5-HT

**Formula**
C₁₀H₁₂N₂O

**PubChem**
[CID 5202](https://pubchem.ncbi.nlm.nih.gov/compound/5202)

## Structure

- An indole ring with a 5-hydroxyl and an ethylamine side chain.

## Clinical notes

- Carcinoid syndrome: tumour serotonin causes flushing, diarrhoea and right-heart valve fibrosis; urinary 5-HIAA is high.
- SSRIs raise synaptic serotonin and are first-line antidepressants.
- Serotonin syndrome (agitation, clonus, fever) appears when serotonergic drugs are combined, for example with MAO inhibitors.
- 5-HT₃ antagonists such as ondansetron prevent chemotherapy-induced vomiting.

## Drugs that act here

- [Buspirone](https://durgical.com/en/pharmacology/buspirone/)

## Pathways

- [Serotonin and melatonin synthesis](https://durgical.com/en/biochemistry/pathways/serotonin-and-melatonin-synthesis/)

## Related conditions

- [Carcinoid syndrome](https://durgical.com/en/conditions/carcinoid-syndrome/)
- [Pellagra (niacin deficiency)](https://durgical.com/en/conditions/pellagra/)
- [Migraine](https://durgical.com/en/conditions/migraine/)

## Related

- [Tryptophan](https://durgical.com/en/biochemistry/molecules/tryptophan/)
- [Melatonin](https://durgical.com/en/biochemistry/molecules/melatonin/)
- [Tetrahydrobiopterin (BH4)](https://durgical.com/en/biochemistry/molecules/tetrahydrobiopterin/)
- [Niacin (vitamin B3)](https://durgical.com/en/biochemistry/molecules/niacin/)
- [Vitamin B6 (pyridoxal phosphate)](https://durgical.com/en/biochemistry/molecules/vitamin-b6/)

## Test yourself

Think of the answer, then tap the question to see it.

## From which amino acid is serotonin made?

Tryptophan, by tryptophan hydroxylase and then aromatic L-amino acid decarboxylase.

## Where is most of the body's serotonin?

In the enterochromaffin cells of the gut, about 90%; platelets carry it in the blood.

## Which urinary product of serotonin is high in carcinoid syndrome?

5-HIAA (5-hydroxyindoleacetic acid).

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Molecules are shown in their neutral form, as the databases deposit them; in the cell the acids and phosphates are ionised.

Educational information, not medical advice. For any symptom, see a health professional.

Sources: [Serotonin (CID 5202)](https://pubchem.ncbi.nlm.nih.gov/compound/5202) (PubChem, NCBI, 2026) · Public domain; [5-HIAA urine test: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/003612.htm) (MedlinePlus, US National Library of Medicine, 2026); Textbook of Biochemistry for Medical Students, 6th ed. (Jaypee Brothers Medical Publishers (Vasudevan DM, Sreekumari S, Vaidyanathan K), 2011).

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Representative model, for educational purposes only.

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Source: https://durgical.com/en/biochemistry/molecules/serotonin/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/biochemistry/molecules/serotonin/