5-Hydroxyindoleacetic Acid (5-HIAA)

If you have been dealing with unexplained flushing, persistent diarrhea, or wheezing, this is the test that can help settle whether a hidden, serotonin-producing tumor is behind it. 5-HIAA (5-hydroxyindoleacetic acid) is the main waste product your body makes when it breaks down serotonin, and it shows up in urine in unusually large amounts when certain tumors are pumping out serotonin behind the scenes.

This is a targeted test, not a routine screen. It is most useful when symptoms point toward a neuroendocrine tumor (NET) or when you already have one and want to track how active it is. Standard blood panels will not catch what this test detects, because routine labs do not measure serotonin metabolism.

What This Test Actually Measures

Your body makes serotonin from the amino acid tryptophan, mostly in cells lining the gut and in the brain. When serotonin is broken down by enzymes called monoamine oxidases, mainly in the liver, lungs, and brain, the leftover molecule is 5-HIAA, which your kidneys clear into urine. So the level in your urine reflects how much serotonin your body has been producing and processing recently.

Healthy people produce a small amount of 5-HIAA from normal serotonin turnover. But certain serotonin-secreting tumors can flood the system, sending urine 5-HIAA to dramatically high levels. That is the signal this test is designed to catch.

Carcinoid Syndrome and Neuroendocrine Tumors

This is the headline use of the test. A 24-hour urine 5-HIAA collection is described as the established biochemical test for diagnosing NETs in people who already have carcinoid syndrome, with reported sensitivity around 100% and specificity of 85 to 90% in that specific group. In plain terms: when the test is elevated in someone with classic symptoms, it is very likely correct, and it rarely misses true cases. Sensitivity is considerably lower (around 35% in one consensus statement) when applied to all NETs broadly, including those without carcinoid symptoms.

Carcinoid syndrome is the cluster of symptoms (flushing, diarrhea, sometimes wheezing) that happens when a serotonin-secreting tumor releases enough serotonin to reach the rest of the body. The tumors themselves are most often found in the small intestine, but can also start in the lung, colon, or appendix. Guidelines recommend ordering 24-hour urine or plasma 5-HIAA as the first biochemical test when these symptoms appear without another clear explanation.

Tracking Tumor Activity and Disease Progression

For people already diagnosed with a NET, 5-HIAA is not just a one-time diagnostic. It can track how active the tumor is over time. In one cohort of NET patients, a shorter doubling time of 24-hour urinary 5-HIAA was linked to a higher risk of disease progression and disease-specific mortality. A faster-rising number suggests the tumor is becoming more active.

Very high urinary 5-HIAA (above ten times the upper limit of normal) has been linked to shorter overall survival in gastrointestinal NETs. However, this association lost independent prognostic value once tumor grade and other markers (chromogranin A, NSE which stands for neuron-specific enolase) were factored in. So 5-HIAA is most useful when read alongside these other measurements, not in isolation. A 2024 systematic review concluded that while elevated 5-HIAA has been associated with worse outcomes in some retrospective studies, the results were inconsistent and the overall evidence was of low to very low quality.

In nonfunctional enteropancreatic NETs, where there are no classic carcinoid symptoms, decreases in elevated urinary 5-HIAA during lanreotide therapy (a long-acting hormone-blocking drug) were associated with longer progression-free survival. The trend matters, not just a single reading.

Carcinoid Heart Disease Risk

Persistently high serotonin levels can damage the heart valves, a condition called carcinoid heart disease. Higher and rising 5-HIAA over time has been linked to this risk. In a 5-year follow-up study of people with small-intestinal NETs, cumulative 5-HIAA showed strong predictive performance, with an AUC (a measure of test accuracy where 1.0 is perfect) of 0.98 in that single-cohort study. Other studies have found NT-proBNP (a heart-strain marker) and plasma 5-HIAA to be comparable or complementary, and broader reviews note the evidence supporting serial 5-HIAA for carcinoid heart disease prediction is still limited.

What this means for you: if you have an active serotonin-producing NET, watching 5-HIAA over months and years, alongside NT-proBNP and echocardiography, can help flag whether your heart is at meaningful risk before symptoms appear.

Other Conditions Where 5-HIAA Shifts

Outside of NETs, urinary 5-HIAA has been studied as a possible signal for several gut and brain-related conditions, though it is not yet used as a standalone clinical test in these settings:

• Irritable bowel syndrome (diarrhea-predominant): higher urinary 5-HIAA correlated with worse gut symptoms and higher anxiety scores in a study of 120 people.
• Lymphocytic colitis: higher levels tracked symptom severity and fell with budesonide treatment.
• Migraine: in 53 people with episodic migraine, lower interictal urinary 5-HIAA was linked to more migraine days, more disability, and higher depressive symptom scores. Lower, not higher, correlated with worse disease.
• Untreated coeliac disease in children: an elevated 5-HIAA to creatinine ratio was found in 8-hour urine collections, which fell after gluten withdrawal in 42 children studied.
• Acute appendicitis: spot urinary 5-HIAA showed only modest accuracy (sensitivity around 44 to 71%, AUC around 0.64) and is not recommended as a standalone test.

Reconciling High Versus Low Findings

You may notice that in carcinoid syndrome, high 5-HIAA is the warning sign, while in migraine and functional constipation, low 5-HIAA seems to track with worse disease. That looks like a contradiction, but it is not. 5-HIAA is a serotonin turnover marker, not a simple good-number or bad-number test. In tumor-driven disease, the system is flooded with serotonin and 5-HIAA shoots up. In other conditions, the issue is that serotonin signaling is underactive or dysregulated, and 5-HIAA can drift down. The same molecule reflects different problems in different contexts. Interpretation depends on why you are testing.

When Results Can Be Misleading

A single urine 5-HIAA can give a misleading picture for several reasons. The most important factors to know:

• Serotonin-rich foods can spike results dramatically: eating 3 to 4 bananas raised urinary serotonin and 5-HIAA 15 to 30-fold for several hours, with effects lasting 8 to 10 hours. Pineapples, kiwi, plums, walnuts, pecans, avocados, tomatoes, and certain cheeses can do similar things because they contain preformed serotonin. Dietary restriction before testing is required.
• Medications can falsely raise the result: acetaminophen, ephedrine, phenobarbital, diazepam, nicotine, caffeine, and glyceryl guaiacolate (an expectorant) have all been reported to interfere. Review all prescription drugs, over-the-counter products, and supplements with your provider before testing.
• Daily cyclic variation: in carcinoid patients, 5-HIAA excretion varied substantially across the day. Overnight urine often best reflects overall excretion and can substitute for a full 24-hour collection.
• Collection errors: 24-hour urine is logistically difficult and prone to over- or under-collection, which throws off the result.
• Kidney function matters: because 5-HIAA is cleared by the kidneys, chronic kidney disease can lower urinary 5-HIAA even when the body is producing a lot.

How This Differs From a Brain Serotonin Marker

A common misconception: urinary 5-HIAA does not reflect brain serotonin activity. In a study comparing urine and cerebrospinal fluid (the liquid around your brain and spinal cord) levels in 29 people, there was no correlation between the two. Urinary 5-HIAA tracks systemic, body-wide serotonin metabolism, dominated by gut sources. It is not a window into your brain chemistry, and it should not be ordered to investigate mood or psychiatric questions.

Tracking Your Trend Over Time

If you have a diagnosed NET or are being monitored for one, a single 5-HIAA reading is far less useful than a trend. Symptoms can fluctuate, diet can shift numbers, and tumor activity changes over months. The doubling time, that is, how fast the number is rising, has been linked to disease progression and mortality risk in some studies.

A reasonable cadence for someone with active disease: a baseline 24-hour urine, a follow-up in 3 to 6 months if treatment is starting or changing, then at least every 6 to 12 months thereafter. For someone with rising values, more frequent monitoring (every 3 months) is justified, especially when watching for carcinoid heart disease. Pair the test with an echocardiogram and NT-proBNP if 5-HIAA stays high or keeps climbing.

What to Do With an Out-of-Pattern Result

A single elevated urinary 5-HIAA is not a diagnosis. It is a signal to investigate further. The standard pathway involves several next steps.

• Confirm with a clean repeat: retest after a strict serotonin-restricted diet for at least 48 hours and after reviewing any medications or supplements (including acetaminophen, diazepam, caffeine, and nicotine) that might shift the result.
• Add complementary biomarkers: chromogranin A and NSE are typically measured alongside 5-HIAA, and serum or plasma 5-HIAA is now considered an equivalent and more convenient alternative for people with normal kidney function. NT-proBNP, a heart-strain marker, is the best available blood test for screening people with carcinoid syndrome for clinically significant carcinoid heart disease.
• Image the tumor: if biochemistry stays elevated, somatostatin receptor PET/CT and cross-sectional imaging (CT or MRI) are used to find and stage the tumor.
• Bring in a specialist: an endocrinologist or oncologist familiar with neuroendocrine tumors should lead the workup. If 5-HIAA stays elevated over time, add an echocardiogram (and NT-proBNP) to screen for carcinoid heart disease.

For someone without symptoms whose 5-HIAA comes back normal: this is reassuring, but a single normal result in the right clinical context (no flushing, no chronic diarrhea, no other risk signals) does not require further chasing. The test is most informative when ordered with a clear question in mind.