5-Hydroxyindoleacetic Acid (5-HIAA)

If you have been getting flushing episodes, persistent unexplained diarrhea, or wheezing that no one can pin down, this is the test that can change the conversation. Urinary 5-HIAA (5-hydroxyindoleacetic acid) is the leftover product your body makes when it breaks down serotonin, and it is the most established lab signal for a group of rare, slow-growing tumors called neuroendocrine tumors that quietly pump out too much serotonin.

The reason this number matters even when you feel mostly fine is that serotonin-producing tumors often grow for years before they are caught, and the serotonin they release can scar the right side of the heart. A 24-hour urine collection that captures every drop of 5-HIAA your body makes in a day is one of the few non-invasive ways to spot this pattern early.

What This Test Actually Measures

5-HIAA is the main breakdown product of serotonin. Your body makes serotonin in the gut, the brain, and certain neuroendocrine cells, then breaks it down through enzymes mostly in the liver and lungs, and finally clears the leftover 5-HIAA through the kidneys into urine. Measuring it in a 24-hour urine sample gives a snapshot of how much serotonin your body has been producing and processing over a full day.

Only a tiny fraction of the protein building block tryptophan in your diet normally becomes serotonin and then 5-HIAA. In healthy people, this leaves a low, stable amount in urine. In someone with a serotonin-secreting tumor, that fraction can rise dramatically, sometimes many times above what a healthy body would produce.

Carcinoid Syndrome and Neuroendocrine Tumors

The classic reason to order this test is suspected carcinoid syndrome, a condition where a serotonin-secreting tumor causes symptoms like flushing, diarrhea, and wheezing. For diagnosing carcinoid syndrome in people with neuroendocrine tumors, 24-hour urinary 5-HIAA has roughly 100% sensitivity (it catches almost everyone with the condition) and 85 to 90% specificity (it correctly clears most people who do not have it). That makes it the standard biochemical test for this diagnosis.

It is also used to monitor people with known neuroendocrine tumors, including small-intestinal, lung, colon, and appendiceal tumors, when 5-HIAA was elevated at diagnosis. The number tends to move with tumor activity, so tracking it over time gives a real-time read on how the disease is behaving.

Carcinoid Heart Disease Risk

This is the finding that most changes how seriously the number should be taken. In people with serotonin-secreting tumors, too much serotonin circulating in the blood can scar the valves on the right side of the heart over time, a condition called carcinoid heart disease.

Higher and persistently elevated urinary or plasma 5-HIAA is strongly linked to the presence of carcinoid heart disease and worse survival. In a 5-year follow-up study of people with small-intestinal neuroendocrine tumors, the cumulative 5-HIAA exposure over time predicted heart involvement with very high accuracy (a discrimination measure of 0.98, where 1.0 would be perfect). The takeaway is that the longer your body is exposed to high serotonin, the more likely heart damage becomes.

Prognosis and Disease Progression

In a study of 371 people with gastrointestinal neuroendocrine tumors, urinary 5-HIAA more than 10 times above the upper limit of normal predicted shorter overall survival. The catch is that once tumor grade and other markers like chromogranin A were factored in, 5-HIAA on its own no longer added independent prognostic information. So a very high number is a red flag, but it is read alongside other tests, not in isolation.

The speed of change matters too. In a study of 90 people with neuroendocrine tumors, a shorter 5-HIAA doubling time (how quickly the level keeps climbing) was linked to higher risk of disease progression and disease-specific mortality. A rapidly rising number is a stronger warning than a single elevated reading.

Other Conditions Linked to 5-HIAA

Several gut and brain conditions show shifts in urinary 5-HIAA, though these are research findings rather than reasons to use the test diagnostically. In a study of 120 people with irritable bowel syndrome, those with diarrhea-predominant IBS had higher urinary 5-HIAA, and the levels tracked with gut symptom severity and anxiety scores. In lymphocytic colitis, higher urinary 5-HIAA correlated with symptom severity and fell with treatment.

Less expected: in a study of 53 people with episodic migraine, urinary 5-HIAA was actually lower between attacks, and lower levels correlated with more migraine days and higher disability. Lower urinary 5-HIAA has also been observed in functional constipation and Alzheimer's disease. These are exploratory findings, but they show 5-HIAA reflects more than just tumor activity.

Reconciling the Mixed Picture

Here is how to hold the high-versus-low evidence together. 5-HIAA is not a simple good-number-bad-number marker. It is a window into serotonin pathway activity. Markedly high levels almost always point toward a serotonin-secreting tumor and the risks that come with it. More modest elevations or reductions reflect how the broader serotonin system is functioning across the gut and brain. The dramatic clinical decisions sit at the very high end. The subtler patterns are research-grade signals.

Spot Use in Acute Appendicitis

Spot (single-sample) urinary 5-HIAA has been tested as a quick screen for acute appendicitis. In a study of 97 people, it had 71% sensitivity and 50% specificity. In a separate study of 70 people, sensitivity was 44% with 81% specificity. Both fall short of standard markers like white blood cell count and CRP (C-reactive protein, a general inflammation marker). It is not recommended as a standalone test for ruling appendicitis in or out.

Tracking Your Trend

A single 5-HIAA reading captures one day. Serotonin output varies through the day, and a study of 26 people with carcinoid tumors found marked cyclic changes in excretion, with overnight collections often best reflecting overall output. The implication is that one number is a starting point, not a verdict.

For people without a known tumor, get a baseline collection so you have your own reference. If anything is elevated or borderline, repeat the test within 3 to 6 months under controlled diet conditions to confirm the pattern. For people with a known neuroendocrine tumor, repeated testing on a regular schedule tracks tumor activity and helps catch heart involvement before it becomes irreversible. Cumulative exposure over years matters more than any one snapshot.

When Results Can Be Misleading

This is a test where preparation matters more than for most labs. A handful of common factors can completely distort the number:

• Serotonin-rich foods: eating 3 to 4 bananas raised urinary serotonin output 15 to 30 fold for several hours, returning to normal after 8 to 10 hours. Pineapples, kiwi, plantains, walnuts, pecans, hickory nuts, avocados, and tomatoes also raise levels. Most labs require avoiding these foods for at least 24 to 72 hours before and during collection.
• Collection errors: the 24-hour urine method is logistically demanding and prone to over-collection or under-collection. Missing even one trip to the bathroom can change the number significantly.
• Daily cycling: serotonin output varies through the day, so a single spot urine can mismeasure what a full 24-hour collection would show.
• Kidney function: in chronic kidney disease, urinary 5-HIAA can be lower than expected even when production is high, because the kidneys are not clearing it efficiently.

Decision Pathway for Out-of-Pattern Results

If your urinary 5-HIAA comes back elevated, the next steps depend on the size and pattern of the elevation. A modestly raised number with potential dietary contamination should be repeated under strict diet control before any further workup. A clearly elevated result, especially one well above the normal range, warrants a full neuroendocrine tumor workup.

That workup typically includes chromogranin A (a broader neuroendocrine tumor marker) and imaging such as somatostatin receptor PET/CT. An endocrinologist or oncologist with neuroendocrine tumor experience is the right specialist to involve. If carcinoid syndrome is confirmed or suspected, a baseline echocardiogram and follow-up cardiac imaging to screen for carcinoid heart disease is standard. The combination of high 5-HIAA plus a positive imaging finding is what drives treatment, not 5-HIAA alone.