This test is mainly used to help detect or monitor carcinoid tumors, which are slow-growing neuroendocrine tumors that often arise in the digestive tract or lungs. These tumors can produce excess serotonin, which gets broken down into 5-HIAA and excreted in urine. Carcinoid syndrome is a condition caused by serotonin-secreting tumors and is one of the key reasons this test is ordered.