Key Insights

• Understand how this test reveals your body’s current biological state—specifically, whether a neuroendocrine tumor is producing excess serotonin and its metabolite, 5-HIAA.
• Identify a clinically relevant biomarker that helps explain flushing, diarrhea, wheezing, or unexplained valvular heart changes in the setting of suspected serotonin-secreting neuroendocrine tumors.
• Learn how genetics, tumor location, and diet may shape 5-HIAA levels, clarifying why some tumors drive strong signals while others remain biochemically quiet.
• Use insights to guide next steps with your clinician, from imaging and pathology confirmation to evaluating treatment response for functional midgut neuroendocrine tumors.
• Track changes over time to monitor disease activity, detect recurrence, or assess how interventions influence serotonin production.
• When appropriate, integrate findings with related panels (e.g., chromogranin A, pancreastatin) and imaging (e.g., somatostatin receptor PET/CT) for a more complete view of tumor biology.

What Is a 5-HIAA Urine Test?

The 5-HIAA urine test measures 5-hydroxyindoleacetic acid, the main breakdown product of serotonin. In serotonin-producing neuroendocrine tumors (NETs) — especially midgut carcinoid tumors — excess serotonin is metabolized in the liver to 5-HIAA and excreted in urine. Most labs use a 24-hour urine collection to capture a full day of production, and results are typically reported as total amount per 24 hours (e.g., mg/24 h) or as a creatinine-corrected ratio. Modern methods such as high-performance liquid chromatography or liquid chromatography–mass spectrometry improve specificity and reduce false signals. Results are compared against lab-specific reference ranges to flag abnormal elevation.

Why it matters: serotonin signaling influences gut motility, vascular tone, and tissue growth. When a NET overproduces serotonin, it can drive the classic “carcinoid syndrome” picture and, over time, fibrotic heart valve changes. Measuring urinary 5-HIAA provides objective evidence of that biological activity. It helps uncover hidden risks, supports diagnosis alongside imaging and pathology, and offers a concrete way to monitor disease behavior and treatment response. In short, it’s a window into how actively a tumor is secreting, both now and across time.

Why Is It Important to Test Your 5-HIAA?

Serotonin-secreting NETs can quietly accelerate the body’s “go” signals in the gut and blood vessels, creating a ripple effect of flushing, loose stools, and occasionally wheezing. Over months to years, persistent exposure can scar heart valves on the right side of the heart. The 5-HIAA urine test translates that invisible chemistry into numbers you and your care team can track. It’s particularly relevant when symptoms suggest carcinoid syndrome, imaging raises suspicion for a midgut NET, or when a known NET needs biochemical monitoring. While not every NET secretes serotonin, those that do often reveal themselves through elevated 5-HIAA.

Big picture: testing turns guesswork into a trend line. Regular 5-HIAA measurement can flag early warning signs, measure response after therapy, and help anticipate complications like carcinoid heart disease when levels remain high. It does not diagnose cancer on its own — tissue and imaging do that — but it shows where your biology stands today and how it adapts with time. That clarity supports smarter prevention, better symptom control, and more confident long-term planning.

What Insights Will I Get From a 5-HIAA Urine Test?

Your report will show a numerical value compared with a reference range, often as total 5-HIAA per 24 hours. “Normal” reflects what’s typical for a general population. “Optimal” may be described by your clinical team as a level consistent with minimal serotonin overproduction in your specific context. Because biology is personal, a mildly elevated or borderline result can be meaningful only when interpreted alongside symptoms, imaging, pathology, and repeat testing.

When 5-HIAA sits comfortably within the reference range, it suggests there isn’t significant serotonin excess. In people with a previously elevated value, a return toward normal may indicate reduced tumor secretory activity — for example, after surgery or medical therapy — though confirmation with imaging and clinical assessment is essential. Day-to-day variation happens, so trends carry more weight than a single number.

Higher values generally point to increased serotonin production from a functional NET. Marked elevations strengthen the case for carcinoid syndrome and may correlate with risk for carcinoid heart disease, a reason many clinicians layer in cardiac monitoring when 5-HIAA stays high. Lower-than-expected levels in someone with clear symptoms can occur if the tumor doesn’t secrete serotonin, if urine collection was incomplete, or if diet and medications affected the assay. Abnormal does not equal disease by itself — it’s a signal to align labs with the clinical story.

Context and limitations matter. Serotonin-rich foods like bananas and avocados can transiently bump readings, and certain medications can interfere with measurement; most labs provide short preparation instructions to reduce false signals. Different assays (HPLC vs LC–MS/MS) and laboratories use different reference intervals, so comparing results over time is best done with the same lab when possible. Finally, 5-HIAA is most sensitive for midgut, serotonin-secreting tumors; foregut and hindgut NETs may be biochemically silent. That’s why your clinician integrates this test with other biomarkers and modern imaging to see the full picture.