Tryptophan is involved in serotonin production via vitamin B6-dependent pathways resulting in the intermediate 5-hydroxytryptophan (5-HTP). 5-HTP is often used as a supplement for serotonin formation instead of tryptophan, which can be quickly metabolized in other pathways.

Serotonin is further metabolized to melatonin via methylation.

Because of these downstream conversions, therapeutic administration of 5-HTP has been shown to be effective for depression, fibromyalgia, binge eating associated with obesity, chronic headaches, and insomnia.

Tryptophan can be alternatively metabolized via the kynurenine pathway to produce various organic acids - kynurenic acid, quinolinic acid, and xanthurenic acid.

Two percent of dietary tryptophan is converted to niacin (vitamin B3) in the liver and deficiencies of vitamin B6, riboflavin, iron, and heme as essential cofactors for enzymes can slow the reaction rate.

    • Hartnup disease is a rare genetic disorder involving an inborn error of amino acid metabolism with symptoms developing in childhood. The intestines cannot properly absorb neutral amino acids and the kidney cannot properly resorb them. This leads to increased clearance of neutral amino acids in the urine, and normal or low levels in the plasma. Tryptophan deficiency is thought to account for the symptoms, since tryptophan converts to vitamin B3. This B3 deficiency causes dermatitis, a characteristic feature of Hartnup disease.