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 5HIAA (5-hydroxyindoleacetic acid), are excreted in the urine.

Normally, serotonin is present in small varying quantities in the blood. Large quantities of serotonin and 5HIAA 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 it used?

Serotonin may be ordered along with, or following, a 24-hour urine 5HIAA test to help diagnose carcinoid tumours. The test may not be needed if other tests have provided the diagnosis. 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 5HIAA test results are normal or near normal.

What does the 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 5HIAA 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 5HIAA is unlikely to have a serotonin-secreting carcinoid tumour. For monitoring of patients with diagnosed carcinoid tumours, platelet serotonin is less useful than 5HIAA.

Common questions

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

It is up to you and your doctor to determine the tests that should be ordered. Serotonin and 5HIAA offer complementary information. In some cases, 5HIAA is preferred because it is more stable and since it is collected for 24 hours, there is more chance of identifying it 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, according to the American Cancer Society, the average age at diagnosis is usually about 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.
 

More information

RCPA Manual: Serotonin platelets

Healthdirect Australia: Carcinoid syndrome