Indicator Explainer
Tryptophan
The least plentiful of all 22 amino acids and an essential amino acid in humans (provided by food), Tryptophan is found in most proteins and a precursor of serotonin. Tryptophan is converted to 5-hydroxy-tryptophan (5-HTP), converted in turn to serotonin, a neurotransmitter essential in regulating appetite, sleep, mood, and pain. Tryptophan is a natural sedative and present in dairy products, meats, brown rice, fish, and soybeans. (NCI04)
This page explains what the indicator is and how results are interpreted in general terms. Reference ranges depend on the laboratory, the method, and your own situation — no single “normal value” applies to everyone. This page is health information, not a diagnosis. For symptoms, treatment, or urgent concerns, contact a healthcare professional.
When it is tested
Source: National Cancer Institute, NCI Thesaurus 26.09d, C29603. NCI definition, reproduced without modification under CC BY 4.0. Source identity and definition checked automatically on 2026-10-20. No individual clinical interpretation or human reviewer is asserted.
Reading results
Reference intervals and interpretation depend on the laboratory, specimen, method and clinical context. This source definition is educational information; it does not establish a diagnosis, product approval or suitability for an individual.
Also Known As
Related Tests
Tests that measure or involve this indicator.
References
- NCI Thesaurus 26.09d — Tryptophan (C29603)
- Source: National Cancer Institute, NCI Thesaurus 26.09d, C29603. NCI definition, reproduced without modification under CC BY 4.0.
- Urinary Metabolites in Tryptophan-Kynurenine Pathway and Risk of CKD Progression in Patients With Type 2 Diabetes. — Am J Kidney Dis, 2026-10-20 (PMID 42901298; literature)
- Vanillin alleviates circadian disruption-induced cognitive impairment: involvement of gut microbiota alterations and tryptophan metabolism. — Food Funct, 2026-10-19 (PMID 42831371; literature)
- Bifidobacterium animalis subsp. lactis C-2 Alleviates DSS-induced Ulcerative Colitis via the Microbiota-tryptophan Metabolism-AhR Pathway. — Probiotics Antimicrob Proteins, 2026-10-24 (PMID 42925394; literature)
- Gut microbiome dysbiosis and alterations in kynurenine pathway of tryptophan metabolism are associated with depression in people with HIV. — Int J Infect Dis, 2026-10-22 (PMID 42913782; literature)