Serotonin acts as both a neurotransmitter in the brain and a hormone throughout the body, influencing mood, sleep, and digestion. While a blood test is available to measure serotonin levels, it is not a standard tool used by healthcare providers to diagnose conditions like depression or anxiety. This test measures the total amount of serotonin circulating in the body, which does not accurately reflect the levels available in the brain to regulate mood.
Understanding Serotonin Location
The body’s serotonin system is divided into central and peripheral systems. Approximately 90% of serotonin is synthesized in the gut by specialized enterochromaffin cells, forming the peripheral system. This peripheral serotonin regulates functions such as bowel movements, appetite, and blood clotting.
Only 1% to 2% of the body’s total serotonin is located in the central nervous system (CNS), where it acts as a neurotransmitter influencing mood and cognition. The brain is protected by the blood-brain barrier, which strictly controls which substances can pass through. Since serotonin cannot easily cross this barrier, the serotonin used for mood regulation in the brain must be produced locally within the CNS.
Due to this physical separation, the concentration of serotonin measured in a blood test reflects activity primarily in the gastrointestinal tract and platelets. Peripheral serotonin levels in the blood do not reliably indicate the amount of serotonin active in the brain’s synapses. Therefore, using a blood test to assess mood or diagnose a psychiatric condition is scientifically unreliable.
When Serotonin Blood Tests Are Used
The clinical application of a serotonin blood test is highly specific, focusing on diagnosing and monitoring a rare type of cancer. The test is primarily used when a healthcare provider suspects a neuroendocrine tumor, particularly a carcinoid tumor. These tumors, which commonly arise in the gastrointestinal tract or lungs, sometimes produce and secrete excessive amounts of serotonin directly into the bloodstream.
This overproduction can lead to Carcinoid Syndrome, which includes flushing of the skin, chronic diarrhea, and wheezing. Elevated serotonin levels in the blood support the diagnosis of a serotonin-secreting tumor and help monitor treatment effectiveness.
The blood test is frequently ordered alongside a urine test that measures 5-hydroxyindoleacetic acid (5-HIAA), the main breakdown product of serotonin. Measuring 5-HIAA in a 24-hour urine sample provides a comprehensive picture of the body’s total serotonin production. This combined testing approach is the standard procedure for diagnosing and tracking these specific tumors. Outside of this context, the blood test holds little diagnostic utility.
Interpreting Peripheral Serotonin Results
The results from a blood serotonin test reflect the complex activity of the peripheral system, not the chemical balance of the brain. A high result is most clinically meaningful when it points toward a serotonin-secreting tumor, especially when accompanied by symptoms of Carcinoid Syndrome.
Several non-disease factors can cause temporary elevations in peripheral serotonin levels. Certain foods rich in tryptophan, the amino acid precursor to serotonin, may temporarily influence blood levels. Additionally, many medications, including selective serotonin reuptake inhibitors (SSRIs), can interfere with how serotonin is stored in platelets, potentially leading to misleading results. Patients are often instructed to avoid these medications and certain foods before the test to ensure the most accurate reading for tumor diagnosis.
Conversely, lower peripheral levels may be associated with rare genetic conditions or specific nutritional deficiencies. Interpreting a single high or low serotonin blood result in isolation is not medically informative for mood disorders and can lead to incorrect self-diagnosis. Any blood test result should only be interpreted by a medical professional who considers it within the narrow context of specific, related clinical conditions.