Also Known As
5-Hydroxy Tryptamine
Formal Name
This article was last reviewed on
This article waslast modified on 14 November 2019.
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 taken from a vein in your arm

Test Preparation Needed?

No test preparation is needed.

On average it takes 7 working days for the blood test results to come back from the hospital, depending on the exact tests requested. Some specialist test results may take longer, if samples have to be sent to a reference (specialist) laboratory. The X-ray & scan results may take longer. If you are registered to use the online services of your local practice, you may be able to access your results online. Your GP practice will be able to provide specific details.

If the doctor wants to see you about the result(s), you will be offered an appointment. If you are concerned about your test results, you will need to arrange an appointment with your doctor so that all relevant information including age, ethnicity, health history, signs and symptoms, laboratory and other procedures (radiology, endoscopy, etc.), can be considered.

Lab Tests Online-UK is an educational website designed to provide patients and carers with information on laboratory tests used in medical care. We are not a laboratory and are unable to comment on an individual's health and treatment.

Reference ranges are dependent on many factors, including patient age, sex, sample population, and test method, and numeric test results can have different meanings in different laboratories.

For these reasons, you will not find reference ranges for the majority of tests described on this web site. The lab report containing your test results should include the relevant reference range for your test(s). Please consult your doctor or the laboratory that performed the test(s) to obtain the reference range if you do not have the lab report.

For more information on reference ranges, please read Reference Ranges and What They Mean.

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, a muscle stimulant), 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.

According to Cancer Research UK, carcinoid tumours are rare, with only 1,200 people diagnosed each year in the UK. Many more of these tumors may exist, but most remain small and do not cause any symptoms. When carcinoid tumours are discovered in asymptomatic patients during surgical procedures performed for other reasons, they are called "incidental" tumours. A small percentage of these tumours may eventually grow large enough to cause obstructions in the intestines or bronchial tubes of the lungs.

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.

Accordion Title
Common Questions
  • How is it used?

    Serotonin may occasionally be requested 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. Monitoring tests may include 5-HIAA and Chromogranin A.

  • When is it requested?

    This test is primarily requested when a patient has symptoms suggestive of a carcinoid tumour.

    Some signs and symptoms include:

    • Flushing of the face and neck (appearance of deep red color, usually with sudden onset)
    • Diarrhoea, nausea, vomiting
    • Rapid heart rate
    • Wheezing, coughing, difficulty breathing

    Serotonin may occasionally be requested as a follow-up test when 5-HIAA test results are normal or near normal.

  • What does the test result mean?

    A significantly increased concentration 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 usually follow an abnormal test result with a request 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 5-HIAA and serotonin 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 concentrations 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 result, including morphine, monoamine oxidase (MAO) inhibitors, methyldopa, and lithium. Patients should always 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 requested with these conditions.

  • Can I choose between the serotonin and 5-HIAA tests?

    Your doctor will normally request measurement of 5-HIAA in a 24 hour urine collection in the first instance, with serotonin measurement being reserved for cases where the 5-HIAA result does not help. Urine 5-HIAA is preferred because it is more stable and, since it is collected for 24 hours, there is more chance of identifying increased 5-HIAA concentrations than excess serotonin that is only released intermittently.

  • Are some people at a higher risk for developing a carcinoid tumour?

    Anyone at any age can develop a carcinoid tumour but the average age at diagnosis is usually quoted as 55 to 65. Patients with a family history of multiple endocrine neoplasia (MEN1), a genetic condition that increases a patient's risk of developing tumours in the endocrine system glands, may be at higher risk for developing a carcinoid tumour.