Also Known As
5-hydroxyindoleacetic acid
Serotonin metabolite
Formal Name
5-hydroxyindoleacetic acid
This article was last reviewed on
This article waslast modified on 16 December 2019.
At a Glance
Why Get Tested?

To help diagnose and monitor treatment for 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; at intervals following treatment

Sample Required?
Test Preparation Needed?

You may be instructed to avoid certain foods and medications prior to this test. Please follow any instructions you are given.

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 5-hydroxyindoleacetic acid (5-HIAA) in the urine. 5-HIAA is a muscle stimulant and the primary metabolite of serotonin, a chemical derived from the amino acid tryptophan. In health, most serotonin is produced by the gastrointestinal (GI) tract where it regulates intestinal movements. The remainder is produced by the central nervous system where it functions to regulate mood, appetite and the wake/sleep cycle. After it has been used by the body, serotonin is broken down in the liver, and its metabolites, including 5-HIAA, are excreted in the urine. Normally, only small amounts of 5-HIAA is present in the urine. However, large quantities of serotonin and 5-HIAA may be produced by some carcinoid tumours.

Carcinoid tumours are typically slow-growing masses. The majority of carcinoid tumours form in the gastrointestinal tract but they can also form at other sites such as in the lungs and the pancreas. Many carcinoid tumours remain small and do not cause any symptoms. A small percentage of carcinoid tumours grow large enough to cause obstructions in the intestines or bronchial tubes of the lungs. Some carcinoid tumours can spread to other organs. The most common site of spread is to the liver.

About 10% of carcinoid tumours, primarily those found in the gastrointestinal tract, will produce enough serotonin to cause symptoms such as flushing of the face, diarrhoea, a rapid heart rate, and wheezing. This collection of symptoms is referred to as the carcinoid syndrome. Such symptoms will usually only appear after the tumour has spread to the liver. The serotonin that causes the carcinoid syndrome may be released continuously or intermittently and can lead to significantly increased quantities of 5-HIAA in the urine.

How is the sample collected for testing?

For the 24-hour urine collection, all urine should be saved for a 24-hour period. It is better to keep sample in a cool dark place. When complete the sample is taken as soon as possible to your GP or the laboratory for preservation. Pre-sample preparation is important for accurate 5-HIAA test results.
For more information, see Common Questions, Is there anything else I should know below and talk to your doctor.

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

Pre-sample preparation is important for accurate 5-HIAA test results. Foods such as avocados, bananas, pineapples, plums, walnuts, tomatoes, kiwi fruit, and aubergine (eggplant) can interfere with 5-HIAA measurement and should be avoided for 3 days prior to and during urine collection. There are also a variety of drugs that can affect the 5-HIAA test. It is important that those being tested talk to their healthcare provider before decreasing or discontinuing any medications.

Accordion Title
Common Questions
  • How is it used?

    5-HIAA may be requested by itself and very occasionally with blood serotonin to help diagnose and monitor carcinoid tumours. A 24-hour urine sample is preferred for the 5-HIAA test because the amount of 5-HIAA in the urine can vary throughout the day. In exceptional situations a random urine sample is sometimes tested, usually along with a urine creatinine level, when a 24-hour sample is not feasible. The random sample is not as accurate, however, and if the excess 5-HIAA is released intermittently, then it may be missed.

  • When is it requested?

    This test is primarily requested when a patient has symptoms suggestive of a carcinoid tumour. It may also be requested at intervals to help monitor the effectiveness of treatment in patients who have been diagnosed with and treated for a serotonin-secreting carcinoid tumour.

  • What does the test result mean?

    A significantly increased concentration of 5-HIAA in a 24-hour urine sample 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 a sample of it examined. The doctor will frequently ask to repeat the sample collection and test following an abnormal test result before requesting 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 concentration of 5-HIAA is normal. The patient may have a tumour that does not secrete serotonin or one that secretes it intermittently. A patient with no symptoms and a normal 24 hour urine excretion of 5-HIAA is unlikely to have a serotonin-secreting carcinoid tumour.

    In patients who are being monitored following treatment for carcinoid tumour, decreasing concentrations of 5-HIAA indicate a response to treatment, while increasing or continued excessive concentrations indicate that the treatment may have not been successful.

  • Is there anything else I should know?

    Foods such as avocados, bananas, pineapples, plums, walnuts, tomatoes, kiwi fruit, aubergine and health food supplements containing 5-hydroxytrytophen can increase 5-HIAA and should be avoided for three days prior to and during urine collection.

    There are also a variety of drugs that can affect the 5-HIAA test. Drugs that can increase 5-HIAA include acetaminophen, caffeine, ephedrine (an ingredient found in some cough medicines), diazepam (Valium), nicotine, glyceryl guaiacolate (an ingredient found in some cough medicines), and phenobarbital. Drugs that can decrease 5-HIAA include aspirin, ethyl alcohol, imipramine, levodopa, MAO inhibitors, heparin, isoniazid, methyldopa, and tricyclic antidepressants. Patients should talk to their doctor before decreasing or discontinuing any medications.

  • Are there other metabolites of serotonin?

    Yes, the other major metabolite is 5-hydroxytryptphol (5-HTOL). This substance is not routinely tested for. 

  • Why do I have to collect my urine for 24 hours?

    The concentration of 5-HIAA in the urine varies throughout the day. By collecting all urine for 24 hours, the amount of 5-HIAA in the urine can be averaged over the entire day. Increases in 5-HIAA will be detected more easily in a 24hr sample than in a single random urine sample.

  • Will my results be accurate if I must continue to take my medication?

    If the drug is one that can increase or decrease the amount of serotonin and 5-HIAA, then your results may be affected. However, it is up to your doctor and you to decide whether or not your medication can be safely stopped prior to and during the test collection. If your drugs must be taken, then your doctor will interpret the test results with this in mind.

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

    Carcinoid tumours are rare. They can affect anyone at any age but are most commonly diagnosed in persons over 60 years.. 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 tumours.