Serotonin Test · Blood

Most of the serotonin in your body has nothing to do with your mood. About 95 percent of it is made in the lining of your gut and stored in platelets, the cell fragments that help your blood clot. A blood test for serotonin does not read your brain chemistry. What it can do is flag one specific problem: a rare kind of hormone-producing tumor that leaks serotonin into your bloodstream and causes flushing, diarrhea, wheezing, and eventually heart damage if it goes unrecognized.

This is a narrow test with a specific job. It is not a window into depression, anxiety, or brain function, despite the popular story about low serotonin and mood. It is most useful when you have symptoms that do not fit a common diagnosis and you want to rule out a carcinoid tumor, or when you already know you have a neuroendocrine tumor and need to track how much serotonin it is putting out.

What Serotonin Actually Is

Serotonin, or 5-HT (5-hydroxytryptamine), is built from the amino acid tryptophan, which you get from food. The body makes it in two anatomically separate systems that do not talk to each other. One system lives in the brainstem and handles central nervous system functions. The other lives in specialized cells of the gut wall called enterochromaffin cells, and it handles gut movement, platelet activity, and blood vessel tone.

Because serotonin cannot cross from blood into brain, the number on a serum test reflects gut and platelet serotonin, not brain serotonin. This is why the test is used for tumors that secrete serotonin peripherally, not for psychiatric conditions.

Why This Test Exists: Carcinoid Syndrome

Serotonin measurement is used to diagnose and monitor carcinoid syndrome, a cluster of symptoms caused by neuroendocrine tumors that secrete serotonin into the bloodstream. Carcinoid syndrome occurs in about 20 percent of neuroendocrine tumors, and in up to 50 percent of metastatic midgut tumors. The classic symptoms are episodic flushing of the face and chest, watery diarrhea, wheezing, and over time, damage to the right side of the heart.

Most clinical workups use a related test, 24-hour urinary 5-HIAA, which measures serotonin's main breakdown product. Urinary 5-HIAA has a sensitivity of about 100 percent and a specificity of 85 to 90 percent for carcinoid syndrome. A serum serotonin test can complement this, and one pilot study found serum serotonin by modern mass spectrometry performed comparably to serum 5-HIAA for detecting serotonin-producing neuroendocrine tumors.

Heart Disease Link

Sustained serotonin excess damages the right-sided heart valves, a condition called carcinoid heart disease. In people with known neuroendocrine tumors, 24-hour urinary 5-HIAA above 300 micromoles per day confers a two to three-fold higher risk of developing or worsening carcinoid heart disease, and a five-fold higher risk of having more than three flushing episodes per day.

A separate observational study in 121 men undergoing coronary angiography reported that whole-blood serotonin at or above 1,000 nmol/L was associated with about 3.4 times higher odds of coronary artery disease, and the association held after adjusting for standard cardiac risk factors (odds ratio 3.8). A smaller study of 132 outpatients with cardiovascular risk factors found the platelet-poor plasma to whole-blood serotonin ratio was independently linked to stable coronary disease. These cardiovascular findings are preliminary and have not been replicated in large prospective cohorts.

Neuroendocrine Tumor Mortality

In people already diagnosed with neuroendocrine tumors, serotonin output predicts survival. A meta-analysis of 12 studies covering 755 patients with neuroendocrine tumors found that for every 10-unit rise in 24-hour urinary 5-HIAA, one-year mortality increased by 11.8 percent. This is why oncologists track 5-HIAA (and sometimes serotonin) serially in people with known disease.

Colon Cancer Signal

A case-control analysis nested in the European EPIC cohort compared 456 people who later developed colon cancer to matched controls. People with detectable circulating serotonin had about twice the odds of developing colon cancer (odds ratio 2.03). Tryptophan, the amino acid serotonin is built from, showed the opposite pattern: higher levels were associated with lower colon cancer risk. This is a single cohort finding and should not be read as established risk on its own.

Depression Is Not What This Test Tells You

A 2023 umbrella review synthesized 17 systematic reviews, meta-analyses, and large genetic studies on the serotonin theory of depression. Two meta-analyses of serotonin's metabolite 5-HIAA (largest n=1,002) showed no association with depression. A meta-analysis of cohort studies (n=1,869) showed no relationship between plasma serotonin and depression. The largest genetic studies (n=115,257 and n=43,165) found no evidence linking serotonin transporter gene variants to depression.

A serum serotonin number will not tell you whether you are depressed, whether an antidepressant is working in your brain, or whether your mood issues stem from a chemical imbalance. If you are dealing with mood symptoms, this is not the right test.

Reference Ranges

Ranges vary dramatically by lab method and specimen fraction, so the most important caveat is this: compare your result only to your own lab's reference range, and track changes within the same lab over time. A few research-reported ranges give rough orientation.

These ranges are drawn from published research. Your lab may use different assays and cutpoints, and numbers are not interchangeable across specimen types or methods. Compare your results within the same lab over time for the most meaningful trend.

For carcinoid syndrome specifically, serum 5-HIAA (serotonin's metabolite, often ordered alongside serotonin) has a proposed optimal cutoff of 139.4 nmol/L, which caught carcinoid syndrome in about 96 out of 100 cases and correctly cleared it in about 88 out of 100 people without it. There is no validated preventive or longevity target for serum serotonin in the research literature.

Why One Reading Is Not Enough

Serum serotonin is unusually sensitive to how and when the blood is drawn. Levels peak about 30 minutes before meals and drop afterward, and they swing substantially over the course of a day, with highest values in early morning and lowest in midafternoon. Fasting blunts these swings. Obesity flattens them further.

On top of that, sample handling is the single biggest source of error in serotonin testing. Platelets store roughly 99 percent of circulating serotonin, and any disturbance during blood draw or processing can release serotonin into the plasma and artificially raise the reading, sometimes by 2.8-fold or more. A 2025 systematic review concluded that most published plasma serotonin values in the medical literature are erroneously high for this reason.

A sensible approach: get a baseline, repeat the test at the same time of day with the same fasting status, and have the draw done by a lab that routinely runs serotonin and knows the handling protocol. If you are tracking a diagnosed condition, retest every 12 weeks to 12 months depending on your situation. If your first result is surprising, retest before acting on it.

When Results Can Be Misleading

Several things can push your serum serotonin up or down without reflecting real disease.

• Sample handling: platelet activation during blood draw or processing can inflate serotonin readings by nearly 3-fold. This is the largest source of false-high results.
• Recent meals and meal timing: carbohydrate-heavy meals can raise plasma serotonin about 4.5-fold, while a protein-heavy meal can drop it to roughly 28 percent of baseline for several hours.
• Serotonin-rich foods in the 48 hours before testing: bananas, pineapple, kiwi, plums, tomatoes, plantains, walnuts, avocados, and eggplant can raise urinary and possibly blood serotonin measurements.
• Medications that interfere with the assay: acetaminophen can produce falsely low readings on some liquid chromatography methods, and drugs including caffeine, nicotine, levodopa, and certain cough medicines can shift 5-HIAA results without reflecting disease.

If you are on an SSRI or SNRI antidepressant, your platelet serotonin will be suppressed by 70 to 90 percent because those drugs block the platelet serotonin transporter. This is a real biological effect of the drug, but it does not reflect brain serotonin activity or treatment response.