5-HIAA or 5-Hydroxyindoleacetic acid is excreted in the urine and is one of the major metabolite serotonins. People who are suffering from carcinoid tumor and neuroendocrine tumor have excess levels of 5-5-HIAA and serotonin, particularly in those with carcinoid syndrome. Carcinoid syndrome is related is ailments like carcinoid tumor, heart disease, hepatomegaly and flushing. A 24-hour urine sample can help in diagnosing carcinoid disease with a very accurate specific measure.