Serotonin

This test measures the amount of serotonin circulating in your blood. Serotonin is a chemical messenger made mainly in the gut and stored in platelets, and it plays roles in intestinal movement, blood vessel tone, and platelet function. A small amount is also produced in the brain, but blood testing does not reflect brain levels.

Clinicians often check blood serotonin when they are looking for evidence of certain neuroendocrine tumors that can release serotonin into the bloodstream. It may be used alongside other tests, such as urine 5-HIAA or imaging, to help clarify a diagnosis or to monitor treatment.

Higher than expected serotonin in blood can be a clue to serotonin-producing neuroendocrine tumors, especially in people with flushing, diarrhea, or wheezing. The test can also help track how well treatment is working over time.

Many common medicines and foods can shift serotonin results, which is why your clinician may ask you to adjust diet or medications before the blood draw if it is safe to do so. This test is not used to diagnose mood disorders, since blood levels do not mirror serotonin activity in the brain.

Your clinician will interpret your result in the context of symptoms, medication use, and other tests. A result that is higher than expected may prompt repeat testing under controlled conditions, a urine 5-HIAA test, or imaging to look for a neuroendocrine source. If you are taking medicines that affect serotonin or recently ate serotonin-rich foods, your team may recommend holding or avoiding them before retesting if medically appropriate.

A result within the expected range is generally reassuring, but it does not rule out all neuroendocrine tumors. A lower or borderline result can occur for nonworrisome reasons, including specimen handling issues or medication effects. Discuss any changes or next steps with your clinician, who will guide further evaluation based on your overall clinical picture.