The 5-HIAA urine test measures 5-hydroxyindoleacetic acid, the main breakdown product of serotonin, in urine. It is used most often when a clinician suspects a serotonin-producing neuroendocrine tumor (NET) or carcinoid syndrome, especially in someone with unexplained flushing, frequent watery diarrhea, wheezing, or signs of carcinoid heart disease. The classic version is a 24-hour urine collection because serotonin production can vary during the day. A high result can support the diagnosis of a serotonin-secreting NET, but it does not locate a tumor or prove cancer by itself. Foods, medicines, collection errors, kidney function, and other conditions can change the result, so preparation and interpretation matter. A normal result also does not rule out every NET because many neuroendocrine tumors do not make enough serotonin to raise urinary 5-HIAA. Results are most useful when interpreted with symptoms, imaging, pathology, and other laboratory findings.
- What it measures: 24-hour urinary 5-HIAA reflects how much serotonin the body has metabolized and is a key biochemical marker for carcinoid syndrome.
- Typical reference range: Many laboratories use a range near 2–9 mg/24 hours, but the correct range is the one printed by the testing laboratory.
- A high result: Marked elevation raises concern for a serotonin-secreting NET, but diet, medicines, and collection problems can cause false-positive results.
- Preparation matters: Certain serotonin-rich foods and interfering medicines may need to be avoided for several days before and during collection, following the laboratory’s instructions.
- Follow-up matters: Very high or rising 5-HIAA in a person with carcinoid syndrome may prompt closer assessment for tumor activity and carcinoid heart disease.
Table of Contents
- What the 5-HIAA Urine Test Measures
- When the Test Is Used
- How to Prepare and Collect a 24-Hour Sample
- Understanding Normal, High, and Changing Results
- False Results and Important Limitations
- How 5-HIAA Fits Into NET Diagnosis and Monitoring
- Follow-Up, Heart Risk, and Next Steps
What the 5-HIAA Urine Test Measures
5-HIAA is short for 5-hydroxyindoleacetic acid. The body makes serotonin from the amino acid tryptophan. After serotonin has done its work, enzymes—especially monoamine oxidase—convert much of it into 5-HIAA. The kidneys then remove 5-HIAA from the blood and excrete it in urine.
That pathway makes urinary 5-HIAA a useful indirect measure of serotonin production. In healthy people, serotonin is involved in normal gut function, nervous-system signaling, and platelet activity. Some neuroendocrine tumors, particularly well-differentiated tumors arising in the small intestine, can produce unusually large amounts of serotonin. When enough serotonin reaches the systemic circulation, it can cause carcinoid syndrome.
Carcinoid syndrome commonly includes episodic flushing and secretory diarrhea. Some people also develop abdominal cramping, wheezing, or blood-pressure changes. Long-term exposure to serotonin and other tumor products can contribute to fibrotic damage of heart valves, known as carcinoid heart disease.
The urine test does not measure the tumor itself. It measures a metabolic product of serotonin. This distinction explains several practical points:
- A small or nonfunctioning NET may produce little or no excess serotonin and can have a normal 5-HIAA result.
- A high 5-HIAA level indicates increased serotonin metabolism, not the location or stage of a tumor.
- A biochemical result must be interpreted alongside symptoms and anatomic or receptor-based imaging.
- Repeated measurements are often more informative than a single number when clinicians are monitoring an established serotonin-secreting NET.
A related 5-HIAA blood test or serum/plasma 5-HIAA measurement is available in some centers. Blood-based testing may be easier than a 24-hour collection, but methods and reference ranges differ, and urine remains a well-established approach for carcinoid syndrome assessment.
When the Test Is Used
The main reason to order a 5-HIAA urine test is to investigate suspected carcinoid syndrome. It is not a general cancer screening test and is not useful for screening people without a clinical reason to suspect a serotonin-secreting NET.
A clinician may consider testing when symptoms suggest excess serotonin secretion, especially when several occur together. Examples include recurrent facial or upper-body flushing, persistent unexplained watery diarrhea, episodic wheezing, or symptoms that occur in attacks without a more common explanation. Testing may also be part of an evaluation after imaging identifies a mass that could represent a neuroendocrine tumor.
The test can serve three broad purposes:
- Support diagnosis. A clearly elevated result in the right clinical setting strengthens suspicion for a serotonin-producing NET and carcinoid syndrome.
- Establish a baseline. Once a functioning NET is diagnosed, a pretreatment value can provide a reference for later comparison.
- Monitor biochemical activity. Serial values can help show whether serotonin production is falling, stable, or increasing during treatment and follow-up.
Urinary 5-HIAA is most relevant to tumors that produce serotonin. It is much less informative for pancreatic NETs that secrete other hormones or for nonfunctioning NETs. Clinicians may use other biochemical tests depending on the suspected syndrome, such as a chromogranin A blood test, although chromogranin A is nonspecific and can be affected by medicines and other illnesses. Symptoms suggesting a different functional NET may lead to tests for gastrin, insulin, glucagon, vasoactive intestinal peptide, or other hormones instead.
A 5-HIAA result should therefore answer a focused question: Is there evidence of excessive serotonin production that fits this person’s symptoms or known NET? It should not be used as a stand-alone yes-or-no cancer test.
How to Prepare and Collect a 24-Hour Sample
Correct preparation is unusually important for 5-HIAA testing because food-derived serotonin and several medicines can distort the measurement. Laboratories use different analytical methods, so the instructions supplied with the collection container should take priority over any general list.
Many laboratories ask patients to avoid certain foods for about three days before the test and through the collection period. Commonly listed foods include bananas, pineapple, plums, kiwi, avocado, eggplant, tomatoes, and walnuts. Some instructions also restrict other serotonin- or tryptophan-rich foods. The exact list can vary with the assay.
Medicines can also affect 5-HIAA. Report all prescription drugs, over-the-counter products, supplements, and cough or pain medicines to the ordering clinician. Acetaminophen and some older medications have been associated with assay interference or altered 5-HIAA excretion. Never stop a necessary medicine on your own simply to prepare for the test. A clinician or laboratory should decide whether a drug should be continued, held, or documented for interpretation.
Typical 24-hour collection steps
The goal is to collect all urine produced during one complete 24-hour period.
- Choose the start time recommended by the laboratory, often after waking.
- Urinate at the start time and do not collect that first specimen. Record the exact time.
- Collect every urination after that into the supplied container for the next 24 hours.
- At exactly the same time the following day, urinate one final time and add that specimen to the container.
- Store the container as instructed. Some collections require refrigeration or a container with preservative.
- Return the sample promptly according to laboratory directions.
Do not discard urine because the container seems full; contact the laboratory for another container. Do not add toilet water, tissue, stool, or other material. If a collection is missed, the safest approach is usually to tell the laboratory rather than submit an incomplete 24-hour specimen as though it were complete.
A complete collection matters because the laboratory reports the total amount excreted over 24 hours. Missing even one substantial void can falsely lower the result. Conversely, collecting beyond the stated period can falsely raise the calculated daily excretion.
Understanding Normal, High, and Changing Results
There is no single universal 5-HIAA reference range. Results depend on the laboratory, analytical method, units, diet restrictions, and whether the result is expressed as a 24-hour total or relative to creatinine. One commonly cited adult range is roughly 2–9 mg per 24 hours, equivalent to about 10–47 micromoles per 24 hours, but a laboratory may use a different upper limit.
| Result pattern | What it may mean | Common next step |
|---|---|---|
| Within laboratory range | No biochemical evidence of major serotonin excess at that time | Reassess whether symptoms and pretest probability still justify additional NET testing |
| Mild elevation | Possible serotonin excess, diet or medication effect, collection issue, or another cause | Review preparation and often repeat under controlled conditions |
| Clear or marked elevation | More concerning for a serotonin-secreting NET when symptoms fit | Clinical evaluation, imaging, and specialist-directed work-up |
| Rising serial values | May indicate increasing biochemical activity in a known functioning NET | Compare with symptoms, imaging, treatment timing, and other markers |
| Falling serial values | May reflect reduced serotonin production after effective therapy | Continue planned disease assessment rather than relying on the marker alone |
A result just above the upper limit is not interpreted the same way as a value several times above it. Mild elevations deserve careful review for diet, medication, and collection factors before they are treated as evidence of a tumor. Marked elevations are more persuasive when the person has typical carcinoid-syndrome symptoms or a known NET.
Units also matter. A result reported in mg/24 hours cannot be compared directly with a threshold stated in µmol/24 hours without conversion. Laboratories may also report 5-HIAA normalized to urine creatinine when a spot urine method is used. Always compare like with like.
For monitoring, the direction and magnitude of change can matter more than whether every value is technically outside the reference range. A clinician may compare results with the same laboratory and method whenever possible to reduce analytical variation.
False Results and Important Limitations
The 5-HIAA urine test is useful, but it has several limitations that can lead to false reassurance or unnecessary alarm.
Diet is a major source of false elevation. Serotonin-rich foods can increase urinary 5-HIAA. If the person ate a restricted food during the preparation period, the ordering clinician should know before interpreting a borderline result.
Medicines can interfere. Some medications can raise or lower measured 5-HIAA or interfere with older assay methods. Modern laboratory techniques reduce some analytical interference, but medication review remains important.
Incomplete collection can falsely lower the result. A 24-hour total assumes every urine specimen was collected. Missed samples, inaccurate start/stop times, spills, or incorrect storage can make the number unreliable.
Kidney function can affect excretion. Because 5-HIAA leaves the body through the kidneys, impaired renal function may complicate interpretation. This is one reason clinicians do not read the result in isolation.
Not all NETs cause carcinoid syndrome. Many NETs are nonfunctioning or secrete substances other than serotonin. A normal 5-HIAA result therefore cannot rule out a neuroendocrine tumor. Likewise, hindgut NETs often do not produce substantial serotonin, while some lung or ovarian NETs can cause systemic hormonal symptoms without the usual liver-metastasis pathway.
Symptoms overlap with common disorders. Flushing and diarrhea can result from menopause, medications, mast-cell disorders, thyroid disease, infections, irritable bowel syndrome, inflammatory bowel disease, food intolerance, anxiety, or many other causes. A high-quality evaluation starts with the overall clinical pattern rather than assuming that every episode of flushing or diarrhea represents carcinoid syndrome.
If a result conflicts strongly with the clinical picture, repeating the test after careful preparation or using another validated biochemical method may be more useful than overinterpreting a single measurement.
A repeat 5-HIAA test can be more informative than reacting to one mildly abnormal value, especially when the first collection was incomplete or the preparation rules were difficult to follow. When results are followed over time, clinicians look for a consistent pattern alongside symptoms and tumor status. A change in diarrhea or flushing does not always track perfectly with urinary 5-HIAA, and symptom severity can be influenced by treatment, diet, other hormones, and non-NET conditions. For that reason, neither symptoms nor the urine marker should be used alone to judge whether a neuroendocrine tumor is progressing.
Units also deserve attention. Laboratories may report 5-HIAA in mg per 24 hours or micromoles per 24 hours and may use different reference limits. The range printed on the patient’s own report is the correct comparison. Converting a value from another laboratory without checking units can make a normal result look abnormal or the reverse.
How 5-HIAA Fits Into NET Diagnosis and Monitoring
A high urinary 5-HIAA level can provide strong biochemical evidence of serotonin excess, but NET diagnosis requires more than a urine marker. The next steps depend on the clinical situation and may include cross-sectional imaging such as CT or MRI, somatostatin-receptor PET imaging, endoscopy, and ultimately tissue diagnosis when appropriate.
Pathology establishes what a mass actually is and provides information such as differentiation and grade. Imaging shows where disease is located and how extensive it may be. Biochemical markers show whether a tumor is producing a measurable secretory product. These tools answer different questions and are complementary rather than interchangeable.
For a patient with established carcinoid syndrome, urinary 5-HIAA may be measured over time to assess biochemical control. A decline after surgery, somatostatin-analog therapy, liver-directed treatment, peptide receptor radionuclide therapy, or other treatment may support a favorable biochemical response. An increase can prompt review for worsening secretory activity or disease progression, but imaging and clinical symptoms remain essential.
It is also helpful to distinguish urinary 5-HIAA from a direct serotonin blood test. Serotonin itself can fluctuate and is influenced by platelet handling, while 24-hour urinary 5-HIAA integrates serotonin metabolism across a full day. For that reason, 5-HIAA has historically been the more established biochemical marker for classic carcinoid syndrome.
Current research is examining blood 5-HIAA and multianalyte molecular tests that may reduce the inconvenience of 24-hour urine collection. These approaches may be useful in selected centers, but assay standardization and clinical context still determine how results should be used.
Follow-Up, Heart Risk, and Next Steps
A high 5-HIAA result should lead to structured follow-up, not panic. The appropriate next step depends on how high the result is, whether preparation was correct, whether symptoms are typical, and whether a NET is already known.
When the elevation is mild, clinicians often review foods, medicines, supplements, collection completeness, and kidney function before repeating the measurement. When the elevation is clearly abnormal and symptoms suggest carcinoid syndrome, referral to a clinician experienced in neuroendocrine tumors is appropriate. Imaging and additional biochemical tests may follow.
In patients with confirmed carcinoid syndrome, persistently high serotonin exposure matters because it is associated with carcinoid heart disease, which most often damages the right-sided heart valves. European guidance has highlighted substantially elevated urinary 5-HIAA—around 300 µmol/24 hours or higher—as one factor that should increase attention to cardiac screening. This is not a stand-alone diagnostic cutoff for heart disease; echocardiography and cardiac biomarkers such as NT-proBNP are used to assess the heart directly.
People with known carcinoid syndrome should promptly report new shortness of breath, ankle or abdominal swelling, reduced exercise tolerance, fainting, or worsening fatigue. These symptoms can have many causes, but they warrant evaluation because advanced valve disease may initially be subtle.
For routine monitoring, consistency improves usefulness. Collect samples with the same preparation rules, use the same laboratory when practical, and note major treatment changes or medication changes. A trend obtained under similar conditions is easier to interpret than isolated results collected in different ways.
The most important principle is that 5-HIAA is a functional marker, not a complete cancer assessment. It can be highly informative when serotonin secretion is part of the disease, but clinical symptoms, imaging, pathology, treatment response, and cardiac evaluation provide the broader picture.
References
- European Neuroendocrine Tumor Society (ENETS) 2022 Guidance Paper for Carcinoid Syndrome and Carcinoid Heart Disease 2022 (Guideline)
- Urinary 5-Hydroxyindolacetic Acid Measurements in Patients with Neuroendocrine Tumor-Related Carcinoid Syndrome: State of the Art 2023 (Review)
- Carcinoid Heart Disease Management: A Multi-Disciplinary Collaboration 2023 (Review)
- European Neuroendocrine Tumor Society (ENETS) 2024 guidance paper for the management of well-differentiated small intestine neuroendocrine tumours 2024 (Guideline)
- Biochemical Markers for Neuroendocrine Tumors: Traditional Circulating Markers and Recent Development-A Comprehensive Review 2024 (Review)
- 5-HIAA urine test 2024
Disclaimer
This article is for general educational purposes and is not a diagnosis or personal medical advice. Reference ranges, preparation rules, and follow-up thresholds vary by laboratory and clinical situation, so discuss your actual 5-HIAA result with the clinician who ordered it. Seek prompt medical care for severe breathing difficulty, fainting, rapidly worsening swelling, or other urgent symptoms.
