Serotonin
At a Glance
Why Get Tested?
To help diagnose a serotonin-secreting carcinoid tumour
When to Get Tested?
When you have symptoms suggestive of a carcinoid tumour such as flushing, diarrhoea, and/or wheezing
Sample Required?
A blood sample drawn from a vein in your arm
The Test Sample
What is being tested?
This test measures the amount of serotonin in the blood. Serotonin is a chemical derived from the amino acid tryptophan. It is produced as needed by the nervous system, mainly the brain, but also by special cells in the bronchial tubes (lungs) and gastrointestinal tract. In the blood, more than 90% of serotonin is found in the platelets. Serotonin helps transmit nerve impulses and constrict blood vessels, is a participant in the wake-sleep cycle, and affects mood. Serotonin is metabolised by the liver and its metabolites, primarily 5-HIAA (5-hydroxyindoleacetic acid), are excreted in the urine.
Normally, serotonin is present in small varying quantities in the blood. Large quantities of serotonin and 5-HIAA may be produced continuously or intermittently by some carcinoid tumours. Carcinoid tumours are slow-growing masses that can form in the gastrointestinal tract, especially in the appendix, and in the lungs. They are one of several types of tumours that arise from cells in the neuroendocrine system - cells that are found in organs throughout the body and that have both nerve and endocrine aspects. The serotonin produced by carcinoid tumours may cause symptoms such as flushing of the face, diarrhoea, a rapid heart rate, and wheezing, especially when the tumour has spread to the liver. This group of symptoms is referred to as the carcinoid syndrome.
How is the sample collected for testing?
A blood sample is obtained by inserting a needle into a vein in the arm.
Is any test preparation needed to ensure the quality of the sample?
No test preparation is needed.
The Test
How is it used?
Serotonin may be ordered along with, or following, a 24-hour urine 5-HIAA test to help diagnose carcinoid tumours. It is not generally used as a monitoring tool to evaluate the effectiveness of treatment or to detect recurrence of a carcinoid tumour. Results may be reported as serum serotonin or as platelet serotonin if the blood platelet count is included in the calculation.
When is it requested?
This test is primarily ordered when a patient has symptoms suggestive of a carcinoid tumour. It may be ordered initially or as a follow-up test when 5-HIAA test results are normal or near normal.
What does the test result mean?
A significantly increased level of serotonin in a patient with carcinoid syndrome symptoms is suggestive but not diagnostic of a carcinoid tumour. In order to diagnose the condition, the tumour itself must be located and biopsied. The doctor will frequently follow an abnormal test result with an order for an imaging scan to help locate any tumour(s) that may be present.
A patient with symptoms may still have a carcinoid tumour even if the concentrations of serotonin and 5-HIAA are normal. The patient may have a tumour that does not secrete serotonin or one that secretes it intermittently. A patient with no symptoms and normal or low levels of serotonin and 5-HIAA is unlikely to have a serotonin-secreting carcinoid tumour.
Is there anything else I should know?
There are a variety of drugs that can affect the serotonin test, including morphine, monoamine oxidase (MAO) inhibitors such as reserpine, methyldopa, and lithium. Patients should talk to their doctor before decreasing or discontinuing any medications.
Serotonin concentrations may be slightly increased in patients with intestinal obstructions, acute myocardial infarction (heart attack), cystic fibrosis, and dumping syndrome. The serotonin test is not usually ordered with these conditions.
Common Questions
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Article Sources
NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used.
