5-Hydroxyindolacetic acid (5-HIAA) is waste from serotonin metabolism, a substance that enables the transmission of nerve influx from one neuron to another. Serotonin is produced primarily in the brain, but small amounts are produced in specialized bronchi and gastrointestinal cells. After use, serotonin is broken down in the liver into 5-HIAA that is excreted in the urine. The results are expressed in micromoles of 5-HIAA per day (µmol/d) and are normally very low.
High levels of 5-HIAA in the urine indicate the presence of a carcinoid tumour, a non-cancer or slow-growing cancer that can form in the digestive tract (2 out of 3 cases) or the lungs. Most carcinoid tumours do not cause symptoms and are found accidentally during surgical procedures for other causes. Rare tumours can become large enough to cause bowel or bronchial obstructions. About 10% of carcinoid tumours will produce enough serotonin to cause the typical symptoms of carcinoid syndrome, including facial flushing, diarrhea, accelerated heart rate, wheezing. Serotonin secretion may be continuous or episodic and a single 5-HIAA negative test result may not be sufficient to definitively eliminate the diagnosis.