A serotonin test measures serotonin or its breakdown product (5‑HIAA) in blood or 24‑hour urine to help detect and monitor conditions causing excess serotonin production, most commonly carcinoid (neuroendocrine) tumors. It’s used when symptoms such as flushing, diarrhea, wheezing or heart issues suggest a serotonin‑secreting tumor, and to track treatment response; it’s not routine for psychiatric diagnoses.

5-HT stands for 5-hydroxytryptamine, the chemical name for serotonin. It’s a monoamine neurotransmitter synthesized from tryptophan that helps regulate mood, sleep, appetite, cognition and gut motility. Although most body serotonin is in the gut, brain serotonin affects emotion and behavior. It works through multiple receptor subtypes, is cleared by the serotonin transporter and broken down by monoamine oxidase.

Normal serotonin varies by test and laboratory. In whole blood (platelet-rich), typical reference ranges are roughly 50–200 ng/mL, though values differ by lab. Plasma and cerebrospinal fluid levels are measured differently, and clinicians often assess the serotonin metabolite 5‑HIAA in urine or CSF. Always compare results to your lab’s reference range and discuss findings with your healthcare provider.

Costs vary by country and test type. A plasma serotonin or 24‑hour urinary 5‑HIAA assay typically runs roughly $30–$300 in the U.S.; private UK testing often costs about £30–£150; in India prices commonly range ₹400–₹2,000. Insurance, hospital testing, and exact assay used can change the patient charge—check with your clinic or local lab for precise, up‑to‑date pricing.

Serotonin deficiency can cause persistent low mood or depression, increased anxiety and irritability, sleep disturbances (insomnia or poor sleep), chronic fatigue, decreased libido, appetite changes or carbohydrate cravings, cognitive issues like poor concentration and memory, gastrointestinal problems (e.g., constipation), emotional instability or social withdrawal, and headaches or heightened stress sensitivity.

High urinary 5‑HIAA suggests increased serotonin breakdown, commonly from serotonin‑secreting neuroendocrine (carcinoid) tumours—especially with liver metastases—and is associated with carcinoid syndrome (flushing, diarrhea, wheeze). False elevations can occur from certain foods (bananas, walnuts, pineapple, avocado, tomatoes, kiwi), serotonergic medications (SSRIs, MAOIs), and renal impairment. Clinical correlation and confirmatory testing are needed.