What is the primary metabolite measured for diagnosing carcinoid syndrome?

The primary metabolite measured for diagnosing carcinoid syndrome is 5-Hydroxyindoleacetic acid (5-HIAA). Carcinoid tumors, which typically arise in the gastrointestinal tract and can secrete large amounts of serotonin, metabolize serotonin primarily into 5-HIAA, which is then excreted in the urine.

In patients with carcinoid syndrome, elevated levels of 5-HIAA in a 24-hour urine sample are a hallmark finding and are used clinically to confirm the diagnosis of these tumors. This elevation occurs because carcinoid tumors often release serotonin directly into the bloodstream, leading to an increase in its metabolite.

Other substances such as serotonin itself may be present but are not commonly measured for diagnosis due to issues with sampling and metabolism. Norepinephrine and histamine do not have the same direct association with carcinoid syndrome and are not utilized for diagnostic purposes in this context. Therefore, measuring the urinary excretion of 5-HIAA is the most reliable and relevant test for diagnosing carcinoid syndrome.