5-HIAA

Unexplained flushing and watery diarrhea can sometimes come from a tumor releasing serotonin. Urinary 5-HIAA can help investigate that possibility, but a result alone cannot establish the diagnosis. Food, medicines, kidney function, and the way urine is collected all affect its interpretation.

The established clinical use is investigating and monitoring serotonin-secreting neuroendocrine tumors, particularly those associated with carcinoid syndrome. Most diagnostic and outcome evidence comes from complete 24-hour urine collections. Evidence for shorter collections and creatinine-adjusted samples supports selected uses, especially follow-up, but does not make every urine format interchangeable.

Where This Molecule Comes From

Your body makes serotonin from tryptophan in food. Most serotonin is produced in the gut. Platelets take it up from the bloodstream, while the brain maintains a separate supply.

Serotonin is broken down through reactions involving monoamine oxidase and aldehyde dehydrogenase, producing 5-HIAA. The amount reaching your urine depends on serotonin production and breakdown, dietary intake, and kidney handling. It is not a direct measurement of serotonin activity in your brain.

What This Urine Result Represents

This test measures urinary 5-HIAA using LC-MS. The method separates compounds and detects them using the mass and electrical charge of their ions. The catalog reports the result in mcg/g.

A per-gram urine result needs its denominator identified on the full report. Many urinary 5-HIAA assays divide the result by urine creatinine to account for dilution. Creatinine comes from muscle metabolism, and its excretion varies with factors including muscle mass. This adjustment reduces the effect of urine concentration but does not turn a single sample into a measurement of total daily output. The unit alone also does not establish whether your collection is random or timed.

Neuroendocrine Tumors and Carcinoid Syndrome

Neuroendocrine tumors arise from cells that can release hormones. Some, particularly small-intestinal tumors, produce large amounts of serotonin. Carcinoid syndrome develops when tumor products reach the general circulation and cause symptoms such as flushing and diarrhea. It often occurs with liver metastases, which allow these substances to bypass the liver's usual clearance of blood arriving from the gut.

Twenty-four-hour urinary 5-HIAA is an established biochemical marker for this syndrome. Its diagnostic accuracy depends on the population, symptoms, collection quality, and laboratory method. It can miss cases, and an elevated result can have causes other than a tumor.

Its sensitivity is much lower when used to look for neuroendocrine tumors of every type. In an older mixed-tumor cohort, urinary 5-HIAA detected about 35% of tumors. Many neuroendocrine tumors release little serotonin. That study found no false positives in its comparison group, but this does not mean the test has perfect specificity in everyday use.

Shorter collections have also been studied. A study involving 80 people with metastatic midgut tumors and 17 controls compared 24-hour urine, a timed overnight collection, and plasma. The overnight collection was pooled urine collected over several hours, so it does not validate an arbitrary single sample.

A separate study compared 136 paired spot and 24-hour samples from 111 people, most with known neuroendocrine tumors. Creatinine-adjusted spot results measured by mass spectrometry tracked 24-hour output closely. This supports their use in follow-up, especially when urinary 5-HIAA is already known to be elevated. Agreement with another urine test is different from independently proving how accurately a test diagnoses cancer.

Carcinoid Heart Disease

Prolonged exposure to excess serotonin can contribute to scarring of the heart valves, especially the tricuspid and pulmonary valves on the right side. This is carcinoid heart disease. Among people with carcinoid syndrome, greater 24-hour urinary 5-HIAA output is associated with a greater risk of valve disease developing or progressing. Those findings cannot be applied directly to a per-gram urine result.

Repeated exposure may matter more than a single reading. A five-year study enrolled 65 people with small-intestinal neuroendocrine tumors and followed 54 prospectively. Cumulative exposure calculated from repeated serum 5-HIAA measurements was associated with valve disease. Only two new cases developed during follow-up, so the estimate needs caution. This was a serum study, not a validation of cumulative exposure calculated from this urine assay.

Survival and Tumor Growth

Higher urinary output often accompanies more extensive serotonin-secreting disease, but its value as an independent predictor of survival is inconsistent. A meta-analysis of 12 studies involving 755 people linked higher 24-hour urinary output with higher estimated annual mortality. It used study-level summaries from varied populations, so it cannot establish an individual prognosis or prove that lowering the marker improves survival.

These studies concern 24-hour excretion, sometimes adjusted for creatinine and measured using older laboratory methods. Their risk estimates have not been validated for this specific mcg/g LC-MS assay.

Hormone release and tumor growth can also diverge. In a study of 48 people, changes in serum 5-HIAA did not reliably distinguish tumors progressing on scans from stable disease. Serum levels were associated with the amount of tumor in the liver. That is evidence about a different specimen, but it helps explain why a biochemical trend must be interpreted alongside imaging and tumor characteristics.

Gut Symptoms, Migraine, and the Brain

Small human studies using creatinine-adjusted morning urine and LC-MS have found higher 5-HIAA in diarrhea-predominant irritable bowel syndrome, with associations between the result and symptom or anxiety scores. Separate research has found lower urinary levels in older adults with functional constipation.

A small study of episodic migraine also found lower creatinine-adjusted urinary levels between attacks, with lower values associated with more headache days. These findings are exploratory. They have not established that using urinary 5-HIAA to choose treatment improves constipation, IBS, or migraine.

Urinary 5-HIAA has no validated role in diagnosing a brain serotonin deficiency or depression. In a small study of psychiatric patients, 24-hour urinary excretion did not correlate with 5-HIAA in spinal fluid. The collection and assay differed from this test, but the finding directly challenges the idea that urinary output measures brain serotonin.

When Results Can Be Misleading

• Food and supplements: Serotonin-rich foods can temporarily increase urinary 5-HIAA. Tryptophan and 5-HTP supplements can also change serotonin production. These are biological changes that LC-MS still detects.
• Kidney function: Reduced kidney function can decrease urinary excretion while increasing circulating 5-HIAA. The effect on a creatinine-adjusted result is more complicated because the denominator can change too.
• Creatinine adjustment: If your result is divided by urine creatinine, differences in creatinine excretion can change the ratio even without an equivalent change in daily 5-HIAA output.
• Medicines: Effects depend on the drug and assay. It is not accurate to assume that all serotonin-acting antidepressants lower urinary 5-HIAA. Some commonly repeated drug warnings originated with older analytical methods.
• Collection and storage: Incomplete timed collections, inconsistent collection times, unsuitable storage, or failure to follow preservation instructions can distort results. A precise instrument cannot correct a poorly collected sample.
• LC-MS interference: Other substances in urine can suppress the instrument's signal or interfere with detection. Proper assay validation and internal controls reduce these problems, but LC-MS is not immune to them.
• Other gut disease: Older timed-urine studies found increased excretion in children with untreated celiac disease. This supports considering other clinical explanations for an unexpected result; it does not make this assay a diagnostic test for celiac disease.

Interpreting a Trend

For a known serotonin-secreting tumor, repeated measurements can show whether hormone output is changing. A rise can reflect increased secretion, changing tumor burden, or a collection or dietary difference. A fall can accompany successful control of secretion without proving that a tumor has shrunk. Comparing the same laboratory method and collection format makes a trend more interpretable.

There is no established urinary target for optimizing mood, migraine, longevity, or general health. In those settings, neither raising nor lowering this number has been shown to improve outcomes.