Tryptophan Unit Converter
Tryptophan Unit Converter
Convert tryptophan between mg/dL and µmol/L, with fasting reference intervals by age — and the two pathways that give the number its meaning, because tryptophan is the precursor of both serotonin and niacin.
Tryptophan converter
mg/dL ⇄ µmol/LTryptophan 1.20 mg/dL on a fasting plasma amino acid profile, read against the adult interval
Formula and conversion factor
mg/dL = µmol/L ÷ 48.9644
- 48.9644
- derived from the molecular weight of L-tryptophan, 204.23 Da — the largest of the standard amino acids, and so the smallest mg/dL to µmol/L factor
- 60 mg : 1 mg
- roughly 60 mg of dietary tryptophan yields 1 mg of niacin equivalents through the kynurenine pathway, with conversion efficiency varying considerably between individuals
- serotonin the other way
- hydroxylation to 5-hydroxytryptophan and then decarboxylation to serotonin. The two pathways compete for the same substrate, which is what links tryptophan to both carcinoid and pellagra
- albumin-bound
- most circulating tryptophan is bound to albumin and laboratories report the total, so the free fraction that crosses into the brain is not what this number shows
Worked example
Tryptophan 1.20 mg/dL on a fasting plasma amino acid profile, read against the adult interval
1.20 × 48.9644 = 58.76 µmol/L, reported as 59 µmol/L
Within the Mayo adult interval of 21–108 µmol/L used here, and within Labcorp's 23.5–93.0 µmol/L
This is a total tryptophan, most of it albumin-bound. It does not show the free fraction, and it does not show how the amino acid is being partitioned between serotonin synthesis and the kynurenine route to niacin
If the clinical question is niacin status or pellagra, tryptophan supply is only half of it — niacin itself, and the dietary history, are what answer it
Two published intervals for total plasma tryptophan
| Source | Adult (µmol/L) | Children (µmol/L) | Younger bands (µmol/L) |
|---|---|---|---|
| Mayo Clinic Laboratories — LC-MS/MS, fasting | 21 – 108 (≥18 y) | 21 – 114 (2–17 y) | 12 – 103 (<24 months) |
| Labcorp — plasma amino acid intervals | 23.5 – 93.0 (>15 y) | 23.9 – 99.3 (2–15 y) | 22.2 – 95.7 (31 d–23 m); 20.0 – 86.0 (0–30 d) |
Two pathways out of tryptophan, and the diseases at the end of each
| Route | What it makes | Where it shows clinically |
|---|---|---|
| Hydroxylation and decarboxylation | 5-hydroxytryptophan, then serotonin, then melatonin | A carcinoid tumour can divert a large share of tryptophan into serotonin. Where the diversion is extensive, a pellagra-like picture can follow because the niacin route is starved |
| Kynurenine pathway | Niacin equivalents — roughly 1 mg of niacin per 60 mg of tryptophan, with wide individual variation | Niacin status depends on dietary niacin and tryptophan supply. Sustained tryptophan shortage raises the risk of pellagra even on an apparently adequate niacin intake |
| Transport into gut and kidney (SLC6A19) | Nothing — but it determines how much tryptophan is absorbed and retained | Hartnup disease: a neutral aminoaciduria with low plasma neutral amino acids including tryptophan, and a pellagra-like rash in a minority, responsive to niacin |
| Binding to albumin | Nothing — but it determines the free fraction | Laboratories report total tryptophan. A low albumin lowers the total without necessarily lowering the free fraction, so a total should be read beside the albumin |
One amino acid, two pathways, two very different diseases
Tryptophan is an essential amino acid and the largest of the standard twenty, with a molecular weight of 204.23 Da. That makes its conversion factor the smallest of the set: 1 mg/dL is 48.96 µmol/L. Most circulating tryptophan is bound to albumin, and a laboratory reports the total, so a tryptophan is worth reading beside the albumin from the same sample.
What makes tryptophan more than a line in a profile is what is made from it. One route hydroxylates it to 5-hydroxytryptophan and decarboxylates that to serotonin, and onward to melatonin. The other, the kynurenine pathway, leads to niacin: roughly 60 mg of dietary tryptophan yields 1 mg of niacin equivalents, though the efficiency of that conversion varies considerably between individuals. Two clinical pictures sit at the ends of those routes. A carcinoid tumour can divert a large share of tryptophan into serotonin, and where the diversion is extensive the niacin route is starved and a pellagra-like picture can follow. In Hartnup disease, a defect of the neutral amino acid transporter in gut and kidney, tryptophan is poorly absorbed and heavily excreted, plasma neutral amino acids run low, and a minority develop a pellagra-like rash that responds to niacin.
The reference intervals behave like the rest of the profile: age-dependent and laboratory-specific. Mayo publishes 21–108 µmol/L for fasting adults, Labcorp 23.5–93.0 µmol/L for anyone over 15, with different bands below that. Published intervals assume a fasting sample separated promptly. Tryptophan is, unusually, one of the stable amino acids in a sample left standing — it changes little at room temperature or on ice — but it still shifts with feeding, so the fasting rule stands.
Read plainly, a plasma tryptophan describes supply and nothing else. It does not show the free fraction, it does not show how the amino acid is being partitioned between serotonin and niacin, and it does not diagnose carcinoid, pellagra or Hartnup disease — those rest on the clinical picture, the urine amino acid pattern, niacin status and a dietary history. A converted tryptophan is one useful measurement in that work, interpreted with the rest of the profile and by the team that ordered it.
Frequently asked questions
How do I convert tryptophan from mg/dL to µmol/L?
Multiply by 48.9644, derived from the molecular weight of tryptophan, 204.23 Da. A tryptophan of 1.20 mg/dL is 59 µmol/L. To go the other way, divide the µmol/L figure by 48.9644.
What is a normal tryptophan level?
Mayo publishes 21–108 µmol/L for fasting adults and Labcorp 23.5–93.0 µmol/L for anyone over 15, with different bands for children and infants. Use the interval your own laboratory prints for that age, on a fasting sample separated promptly.
How is tryptophan related to niacin?
Tryptophan is converted to niacin through the kynurenine pathway — roughly 60 mg of tryptophan yields 1 mg of niacin equivalents, with wide individual variation. That is why niacin status depends on tryptophan supply as well as dietary niacin, and why sustained tryptophan shortage raises the risk of pellagra.
Why does tryptophan connect carcinoid and pellagra?
Because the same amino acid feeds both serotonin and niacin. A carcinoid tumour can divert a large share of tryptophan into serotonin, and where that diversion is extensive the niacin route is starved, so a pellagra-like picture can follow. The diagnosis is not made on a plasma tryptophan.
What happens to tryptophan in Hartnup disease?
The neutral amino acid transporter in the gut and kidney fails, so tryptophan and the other neutral amino acids are poorly absorbed and heavily excreted in the urine. Plasma levels run low, most people are well, and a minority develop a pellagra-like rash that responds to niacin.
Related calculators
References
- National Institutes of Health, Office of Dietary Supplements. Niacin — Fact Sheet for Health Professionals: 60 mg of tryptophan yields 1 mg of niacin equivalents; pellagra; Hartnup disease and tryptophan transport. Accessed 2026.
- Davis JS, Darcy CJ, Piera K, et al. Ex-vivo changes in amino acid concentrations from blood stored at room temperature or on ice: implications for arginine and taurine measurements. BMC Clin Pathol. 2009;9:10.
- Mayo Clinic Laboratories. Test ID: AAQP — Amino Acids, Quantitative, Plasma. Liquid chromatography tandem mass spectrometry. Reference values are for fasting patients, in three age bands (under 24 months, 2–17 years, 18 years and over). Accessed 2026.
- Labcorp. Plasma Amino Acid Reference Intervals — four age bands from 0–30 days to over 15 years, µmol/L. Accessed 2026.
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.