What is Neurotransmitter Metabolites — HVA, 5-HIAA, DOPAC, Quinolinic Acid, Kynurenic Acid?
Urinary neurotransmitter catabolites provide metabolic evidence of monoamine and tryptophan pathway activity downstream of central and peripheral synthesis. HVA is the terminal dopamine metabolite — produced by sequential MAO-A and COMT catabolism — excreted primarily renally; it reflects combined central and peripheral dopamine catabolism rather than synaptic dopamine activity directly. 5-HIAA is the terminal serotonin metabolite via MAO-A — its urinary elevation is diagnostically established for carcinoid tumour and neuroendocrine tumour secreting serotonin, and in functional medicine is used as a proxy for peripheral serotonin turnover and tryptophan flux. Quinolinic acid and kynurenic acid are produced through the kynurenine pathway — the major metabolic route for dietary tryptophan (approximately 95% of tryptophan). Quinolinic acid is an NMDA receptor agonist and neurotoxin produced by activated microglia and macrophages; elevated urinary quinolinic acid is a marker of neuroimmune activation, tryptophan pathway dysregulation, and excitotoxic burden.