5-Hydroxyindoleacetic acid (5HIAA) is a breakdown product of serotonin that is excreted in the urine. Serotonin is a hormone found at high levels in many body tissues. Serotonin and 5HIAA are produced in excess amounts by carcinoid tumors, and levels of these substances may be measured in the urine to test for carcinoid tumors.

5-Hydroxyindoleacetic acid is found to be associated with:

- Aromatic l-amino acid decarboxylase deficiency an inherited disorder that affects the way signals are passed between certain cells in the nervous system.)

- Celiac disease (an autoimmune disorder where the ingestion of gluten leads to damage in the small intestine)

- Sepiapterin reductase deficiency (a condition characterized by movement problems)

- What are normal levels?

Normal results are 3 to 15 milligrams (mg) of 5-HIAA over 24 hours.

Women generally have lower levels than men.

Numerous drugs affect the excretion of 5-HIAA by different mechanisms, including increased serotonin synthesis, metabolism, and release and inhibition of uptake. The following medications can interfere with 5-HIAA results. Proper preparation before the test is essential. Medication and dietary restrictions are required to prevent false-positive results. If clinically feasible, discontinue the following medications at least 48 hours before specimen collection. 

- Acetaminophen (Tylenol or generic versions)

- Aspirin

- Antihistamines

- Cough syrups

- Cold and flu medications

One should also avoid caffeinated beverages such as tea and coffee or caffeinated foods for 48 hours before and during specimen collection.

Certain medicines can raise or lower your 5-HIAA levels. These include:

- Aspirin

- Acetaminophen

- Ethyl alcohol

- Some antidepressants

- Cough medicines or antihistamines

- Medicines to relax muscles

References:

- Urinary sampling for 5HIAA and metanephrines determination: revisiting the recommendations, https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5527357/

- Guidelines for the diagnosis and management of carcinoid tumours. Part 1: the gastrointestinal tract. A statement from a Canadian National Carcinoid Expert Group. (https://www.ncbi.nlm.nih.gov/pubmed/17576444)

- A single fasting plasma 5-HIAA value correlates with 24-hour urinary 5-HIAA values and other biomarkers in midgut neuroendocrine tumors (NETs). (https://www.ncbi.nlm.nih.gov/pubmed/23160483)

- Aromatic l-amino acid decarboxylase deficiency, https://ghr.nlm.nih.gov/condition/aromatic-l-amino-acid-decarboxylase-deficiency

- WHAT IS CELIAC DISEASE?, https://celiac.org/celiac-disease/understanding-celiac-disease-2/what-is-celiac-disease/

- Sepiapterin reductase deficiency, https://ghr.nlm.nih.gov/condition/sepiapterin-reductase-deficiency