A 24-hour urine test for 5-hydroxyindoleacetic acid (5-HIAA) or Serotonin measures the amount of serotonin in your body by measuring the amount of 5-HIAA in your urine
A doctor may order this test to diagnose or monitor neuroendocrine tumors, which can secrete serotonin. Carcinoid tumors, which can occur in the lungs, intestinal tract, appendix, and other tissues, can cause higher levels of 5-HIAA in your urine.