Patient Test Information


Also known as:

5-Hydroxytryptamine; 5-HT

Formal name:


Related tests:

5-HIAA; Chromogranin A

Board approvedAll content on Lab Tests Online has been reviewed and approved by our Editorial Review Board.

Why Get Tested?

To help diagnose a carcinoid tumor that produces serotonin

When to Get Tested?

When you have symptoms suggestive of a carcinoid tumor, such as flushing, diarrhea, and/or wheezing

Sample Required?

A blood sample drawn from a vein in your arm

Test Preparation Needed?


How is it used?

A serotonin test may be used to help diagnose carcinoid tumors. It may be ordered along with, or following, a 24-hour urine 5-HIAA test.

Carcinoid tumors are slow-growing masses that can form in the gastrointestinal tract (especially in the appendix) and in the lungs, although they may affect other organs as well. They are one of several types of tumors 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 test is not generally used as a monitoring tool to evaluate the effectiveness of treatment or to detect recurrence of a carcinoid tumor. Chromogranin A and 5-HIAA may be used for this purpose.

When is it ordered?

This test is primarily ordered when a person has symptoms suggestive of a carcinoid tumor.

Some signs and symptoms include:

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

This test 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 person with carcinoid syndrome symptoms is suggestive but not diagnostic of a carcinoid tumor. In order to diagnose the condition, the tumor itself must be located, biopsied, and examined by a pathologist. The healthcare practitioner will frequently follow an abnormal test result with an order for an imaging scan to help locate any tumor(s) that may be present.

Someone may still have a carcinoid tumor even if the concentrations of serotonin and 5-HIAA are normal. Some carcinoid tumors do not produce serotonin or only produce it intermittently.

A person with no symptoms and normal levels of serotonin and 5-HIAA is unlikely to have a serotonin-secreting carcinoid tumor.

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. People should talk to their healthcare provider before decreasing or discontinuing any medications.

Serotonin concentrations may be slightly increased in those with intestinal obstruction, acute myocardial infarction (heart attack), cystic fibrosis, and dumping syndrome. The serotonin test is not usually ordered with these conditions.

What is being tested?

Serotonin is a chemical substance that transmits messages between nerve cells. 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. More than 90% of serotonin in the blood 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 metabolized by the liver and its metabolites, primarily 5-HIAA (5-hydroxyindoleacetic acid), are eliminated in the urine.

Normally, serotonin is present in small amounts in the blood. Large quantities of serotonin and 5-HIAA may be produced continuously or intermittently by some carcinoid tumors. Carcinoid tumors are slow-growing masses that can form in the gastrointestinal tract (especially in the appendix) and in the lungs, although they may affect other organs as well. They are one of several types of tumors that arise from cells in the neuroendocrine system. These cells, which secrete hormones in response to signals from the nervous system, are found in organs throughout the body. The serotonin produced by carcinoid tumors may cause flushing of the face, diarrhea, a rapid heart rate, and wheezing, especially when the tumor has spread to the liver. This group of signs and symptoms is referred to as the carcinoid syndrome.

According to the American Cancer Society, there are about 8,000 gastrointestinal and 4,000 lung carcinoid tumors diagnosed each year in the United States. Many more of these tumors may exist, but most remain small and do not cause any symptoms. When carcinoid tumors are discovered in asymptomatic patients during surgical procedures performed for other reasons, they are called "incidental" tumors. A small percentage of these tumors 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.

NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.

Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.

Is any test preparation needed to ensure the quality of the sample?

No test preparation is needed.

  1. Should I have both the serotonin and 5-HIAA tests performed?

    Serotonin and 5-HIAA offer complementary information. In some cases, 5-HIAA is preferred because it is more stable and, since it is collected for 24 hours, there is more chance of detecting increased 5-HIAA than in identifying excess serotonin that is only released intermittently. Talk to your healthcare practitioner about which tests are appropriate for your condition. 

  2. Are some people at a higher risk for developing a carcinoid tumor?

    Anyone at any age can develop a carcinoid tumor but, according to the American Cancer Society, the average age at diagnosis is usually about 55 to 65. People with a family history of multiple endocrine neoplasia (MEN1), a genetic condition that increases a person's risk of developing tumors in the endocrine system glands, may be at higher risk for developing a carcinoid tumor.

  3. How does a healthcare practitioner locate the carcinoid tumor?

    This is usually accomplished through the use of imaging scans such as x-rays, computed tomography (CT), or magnetic resonance imaging (MRI). In some cases, surgery is required to find the tumor. For more on these imaging tests, visit

  4. How does the healthcare practitioner tell whether a tumor is benign or cancerous?

    In order to determine whether the tumor is benign or cancerous, the healthcare practitioner will need to perform a biopsy or remove the tumor surgically. The tumor is sent to the laboratory and a pathologist will examine the tumor cells under the microscope. (For more, see the article on Anatomic Pathology.)