Serotonin, also known as 5-hydroxytryptamine (5-HT), is a chemical messenger that acts as both a neurotransmitter and a hormone in the human body. It transmits signals between nerve cells in the brain and throughout the peripheral nervous system. Serotonin plays a wide-ranging role in numerous bodily functions, including regulating mood, sleep patterns, digestion, appetite, and even wound healing and bone health. Imbalances, whether too high or too low, can disrupt these functions and contribute to various health concerns.
Reasons for Serotonin Testing
Healthcare providers typically order serotonin level tests when there’s suspicion of specific medical conditions rather than for general mood disorders. A primary reason for testing is to investigate carcinoid syndrome, a group of symptoms caused by neuroendocrine tumors. These slow-growing tumors, often found in the gastrointestinal tract or lungs, produce excessive serotonin. Symptoms like severe skin flushing, chronic diarrhea, rapid heart rate, and wheezing might prompt a doctor to consider serotonin testing to confirm the presence of these tumors.
It is important to note that blood serotonin levels do not accurately reflect serotonin levels in the brain. Therefore, peripheral serotonin tests are generally not recommended for diagnosing or managing mood disorders such as depression or anxiety.
Methods of Serotonin Testing
Doctors employ several methods to measure serotonin levels or its metabolic byproducts in the body, each utilizing different sample types. Blood tests are a common approach, where samples can be taken to measure either plasma serotonin or whole blood serotonin. Most circulating serotonin is contained within platelets, so whole blood measurements often correlate well with platelet levels. For blood tests, preparation may involve fasting or avoiding certain medications that could interfere with results.
A more frequently used method for diagnosing serotonin-producing tumors is the 24-hour urine test for 5-hydroxyindoleacetic acid (5-HIAA). 5-HIAA is the primary breakdown product of serotonin, and its levels in urine provide an indirect measure of serotonin production over a full day. This test often requires patients to collect all urine over a 24-hour period and adhere to strict dietary and medication restrictions beforehand. The 24-hour urine collection offers a more accurate reflection of the rate at which serotonin is being produced compared to a single blood sample.
Cerebrospinal fluid (CSF) testing can also measure serotonin levels, offering a direct indication of serotonin activity within the brain. This method is more invasive, requiring a lumbar puncture, and is typically reserved for specific neurological investigations rather than routine diagnostic purposes. While CSF testing provides insight into brain serotonin, it is not a standard general test for peripheral serotonin assessment.
Interpreting Serotonin Test Results
Interpreting serotonin test results involves comparing the measured levels to established normal ranges, though these ranges can vary between laboratories. For blood serotonin, normal ranges are often cited as 50 to 200 ng/mL, while 5-HIAA in a 24-hour urine sample generally falls between 2 to 9 mg/24h. Elevated serotonin levels, particularly in conjunction with symptoms, strongly suggest the presence of a carcinoid tumor or carcinoid syndrome. For instance, individuals with carcinoid syndrome might show blood serotonin levels significantly higher, sometimes ranging from 790 to 4,500 ng/mL.
While high levels are often indicative of tumors, normal results do not always rule out a carcinoid tumor, as some tumors do not produce serotonin or do so intermittently. All test results must be interpreted by a healthcare professional, considering a patient’s clinical symptoms, medical history, and other diagnostic findings.
Factors Influencing Serotonin Levels and Test Accuracy
Several factors can influence serotonin levels and the accuracy of test results. Dietary intake of foods rich in serotonin precursors, such as bananas, pineapples, walnuts, avocados, and plums, can temporarily elevate levels and potentially lead to false positives, especially with 5-HIAA urine tests. Patients are typically advised to avoid these foods for a period, often 24 to 72 hours, before testing.
Medications also play a significant role in affecting test accuracy. Certain drugs, including selective serotonin reuptake inhibitors (SSRIs), monoamine oxidase inhibitors (MAOIs), lithium, methyldopa, and some cough medicines containing dextromethorphan, can alter serotonin levels or its metabolism. Patients may need to discontinue these medications, under medical guidance, for a specified period before sample collection. Proper sample collection and handling, such as immediate freezing for some blood samples or refrigeration for urine collections, are also important to ensure result reliability.