Limitations 5-Hydroxyindoleacetic acid
5-hiaa may normal nonmetastatic carcinoid tumor , may normal carcinoid syndrome, particularly in subjects without diarrhea, because patients carcinoid syndrome excrete nonhydroxylated indolic acids. the production , metabolism of serotonin, , 5-hiaa, dependent upon tissue of origin of tumor. tumors midgut cells, such ileal carcinoid contain , release large quantities of serotonin. these amounts may not reflected in amount of 5-hiaa in urine, because little metabolized. foregut tumors lack decarboxylase enzyme necessary convert 5-hydroxytryptophan serotonin, resulting in minimal no elevation in urinary 5-hiaa levels. tumors derived hindgut cells (rectal carcinoid) produce excess serotonin or 5-hiaa. of 75 patients carcinoid tumors, 75% had above normal urinary 5-hiaa excretion , 64% had above normal serotonin excretion.