5-HIAA (5-hydroxyindoleacetic acid)
The main breakdown product of serotonin, measured in a 24-hour urine collection. Its main use is to detect and monitor carcinoid tumours — neuroendocrine tumours that pour out serotonin and cause flushing and diarrhoea. Certain foods and medicines must be avoided first to avoid false results.
What it measures
5-HIAA is the main substance the body makes when it breaks down serotonin, a signalling chemical. It is excreted in the urine, so measuring it in a 24-hour urine collection reflects how much serotonin the body has been producing. Its clinical role is quite specific: it is the key biochemical test for carcinoid tumours — a type of neuroendocrine tumour, most often arising in the gut, that can secrete large amounts of serotonin. When such a tumour is active, especially once it has spread to the liver, the excess serotonin causes carcinoid syndrome (flushing, diarrhoea, wheezing and, over time, effects on the heart), and 5-HIAA in the urine rises. The test is used both to help make the diagnosis and to monitor the tumour over time. A crucial practical point is that many everyday foods rich in serotonin or its building blocks — bananas, pineapple, avocado, walnuts, tomatoes and others — and several medicines can falsely raise or alter 5-HIAA, so a specific diet and drug list must be avoided for a few days before and during the collection. It is used alongside chromogranin A and imaging.
What a high value can mean
- Carcinoid tumour / carcinoid syndrome — the key use; excess serotonin from a neuroendocrine tumour.
- Serotonin-rich foods eaten before the test — bananas, pineapple, avocado, walnuts, tomatoes; a common cause of a false high.
- Certain medicines — some can raise it; a drug review is part of interpreting the result.
- Malabsorption conditions (e.g. coeliac disease) — occasionally raise it modestly.
What a low value can mean
- Normal or low — the expected result; makes an active serotonin-producing carcinoid tumour unlikely.
- A falling level during treatment — can indicate the tumour is responding.
When to discuss with a doctor
Urinary 5-HIAA is a targeted specialist test, ordered mainly when carcinoid syndrome is suspected — for example unexplained flushing with diarrhoea — or to monitor a known carcinoid tumour. Its accuracy depends heavily on preparation: the serotonin-rich foods and interfering medicines must be avoided as instructed for a few days before and during the 24-hour urine collection, or the result can be falsely high. A genuinely raised level, interpreted with chromogranin A and imaging, points toward a carcinoid tumour that needs specialist management. Mediora.AI can surface a raised 5-HIAA from your results and flag the carcinoid pattern for review, but confirming the diagnosis and its treatment is a specialist assessment — the interpretation belongs with your doctor.