Serotonin
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- Also Known As:
- 5-Hydroxytryptamine
- 5-HT

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At a Glance
Why Get Tested?
To help diagnose a carcinoid tumor that produces serotonin. Carcinoid tumors are slow-growing masses that can mainly form in the digestive tract (especially in the appendix) and in the lungs.
When To Get Tested?
When you have symptoms suggestive of a carcinoid tumor, such as flushing, diarrhea, and/or wheezing
Sample Required?
A blood sample drawn from a vein in your arm
Test Preparation Needed?
You may be instructed to avoid certain foods and medications prior to this test.
You may be able to find your test results on your laboratory’s website or patient portal. However, you are currently at Testing.com. You may have been directed here by your lab’s website in order to provide you with background information about the test(s) you had performed. You will need to return to your lab’s website or portal, or contact your healthcare practitioner in order to obtain your test results.
Testing.com is an award-winning patient education website offering information on laboratory tests. The content on the site, which has been reviewed by laboratory scientists and other medical professionals, provides general explanations of what results might mean for each test listed on the site, such as what a high or low value might suggest to your healthcare practitioner about your health or medical condition.
The reference ranges for your tests can be found on your laboratory report. They are typically found to the right of your results.
If you do not have your lab report, consult your healthcare provider or the laboratory that performed the test(s) to obtain the reference range.
Laboratory test results are not meaningful by themselves. Their meaning comes from comparison to reference ranges. Reference ranges are the values expected for a healthy person. They are sometimes called “normal” values. By comparing your test results with reference values, you and your healthcare provider can see if any of your test results fall outside the range of expected values. Values that are outside expected ranges can provide clues to help identify possible conditions or diseases.
While accuracy of laboratory testing has significantly evolved over the past few decades, some lab-to-lab variability can occur due to differences in testing equipment, chemical reagents, and techniques. This is a reason why so few reference ranges are provided on this site. It is important to know that you must use the range supplied by the laboratory that performed your test to evaluate whether your results are “within normal limits.”
For more information, please read the article Reference Ranges and What They Mean.
What is being tested?
Serotonin is a chemical substance that transmits messages between nerve cells. This test measures the amount of serotonin in the blood.
Serotonin is a chemical derived from the amino acid tryptophan. It is produced as needed by the nervous system, mainly the brain, but also by special cells in the bronchial tubes (lungs) and gastrointestinal tract. More than 90% of serotonin in the blood is found in the platelets.
Serotonin helps transmit nerve impulses and constrict blood vessels, is a participant in the wake-sleep cycle, and affects mood. Serotonin is metabolized by the liver and its metabolites, primarily 5-HIAA (5-hydroxyindoleacetic acid), are eliminated in the urine.
Normally, serotonin is present in small amounts in the blood. Large quantities of serotonin and 5-HIAA may be produced continuously or intermittently by some carcinoid tumors. Carcinoid tumors are slow-growing masses that can form in the gastrointestinal tract (especially in the appendix) and in the lungs, although they may affect other organs as well. They are one of several types of tumors that arise from cells in the neuroendocrine system. These cells, which secrete hormones in response to signals from the nervous system, are found in organs throughout the body. The serotonin produced by carcinoid tumors may cause flushing of the face, diarrhea, a rapid heart rate, and wheezing, especially when the tumor has spread to the liver. This group of signs and symptoms is referred to as the carcinoid syndrome.
According to the American Cancer Society, there are about 8,000 gastrointestinal and between 2,000 and 4,500 lung carcinoid tumors diagnosed each year in the United States. Many more of these tumors may exist, but most remain small and do not cause any symptoms. When carcinoid tumors are discovered in asymptomatic patients during surgical procedures performed for other reasons, they are called “incidental” tumors. A small percentage of these tumors may eventually grow large enough to cause obstructions in the intestines or bronchial tubes of the lungs.
Is any test preparation needed to ensure the quality of the sample?
Test preparation is important for accurate serotonin test results. Foods rich in serotonin such as avocados, bananas, pineapples, plums, cantaloupes, grapefruits, plantains, melons, kiwi fruits, walnuts, hickory nuts, butternuts, pecans, tomatoes, and eggplants can interfere with serotonin measurement and should be avoided for 3 days prior to and during sample collection.
There are also a variety of drugs that can affect the metabolism of serotonin and the test itself. It is important that you talk to your healthcare provider before decreasing or discontinuing any medications.
Some laboratories request for overnight fasting prior to collection of blood samples.
Common Questions
View Sources
Sources Used in Current Review
Serotonin, Blood. Mayo Clinic Laboratories. Available online at https://www.mayocliniclabs.com/test-catalog/Clinical+and+Interpretive/84373. Accessed September 2020.
Carcinoid Syndrome. National Organization for Rare Disorders. Available online at https://rarediseases.org/rare-diseases/carcinoid-syndrome/. Accessed September 2020.
Tebbie, Cameron K. (2019 September) Carcinoid Tumor. Medscape. Available online at https://emedicine.medscape.com/article/986050-overview. Accessed September 2020.
(2018 September 24) Key Statistics About Gastrointestinal Carcinoid Tumors. American Cancer Society. Available online at https://www.cancer.org/cancer/gastrointestinal-carcinoid-tumor/about/key-statistics.html. Accessed September 2020.
(2020 January 8) About Lung Carcinoid Tumors. American Cancer Society. Available online at https://www.cancer.org/content/dam/CRC/PDF/Public/8722.00.pdf. Accessed September 2020.
(2014 February 6) 5-Hydroxyindoleacetic Acid (5-HIAA). Medscape. Available online at https://emedicine.medscape.com/article/2089202-overview. Accessed September 2020.
Serotonin. University of Rochester Medical Center. Available online at https://www.urmc.rochester.edu/encyclopedia/content.aspx?contenttypeid=167&contentid=serotonin. Accessed September 2020.
Sources Used in Previous Reviews
Wu, A. (2006). Tietz Clinical Guide to Laboratory Tests, Fourth Edition. Saunders Elsevier, St. Louis, Missouri. Pp 982-983.
Thomas, Clayton L., Editor (1997). Taber’s Cyclopedic Medical Dictionary. F.A. Davis Company, Philadelphia, PA [18th Edition]. Pp 1745.
Van Voorhees, B. W. (2007 May 17, Updated). Serum serotonin level. MedlinePlus Medical Encyclopedia [On-line information]. Available online at http://www.nlm.nih.gov/medlineplus/ency/article/003562.htm. Accessed on 11/1/07.
Nanda, R. (2006 September 11, Updated). Carcinoid syndrome. MedlinePlus Medical Encyclopedia [On-line information]. Available online at http://www.nlm.nih.gov/medlineplus/ency/article/000347.htm. Accessed on 11/1/07.
(2007 September, Reviewed). Carcinoid Tumors. ARUP Consult [On-line information]. Available online through http://www.arupconsult.com. Accessed on 11/1/07.
(© 2007). Serotonin, Whole Blood. ARUP’s Laboratory Test Directory [On-line information]. Available online at http://www.aruplab.com/guides/ug/tests/0080395.jsp. Accessed on 11/1/07.
(© 2007). Serotonin, Serum. ARUP’s Laboratory Test Directory [On-line information]. Available online at http://www.aruplab.com/guides/ug/tests/0080397.jsp. Accessed on 11/1/07.
(2006 May 1, Updated). Tryptophan. MedlinePlus Medical Encyclopedia [On-line information]. Previously available online at http://www.nlm.nih.gov/medlineplus/print/ency/article/002332.htm. Accessed on 11/1/07.
Santacroce, L. and Balducci, L. (2010 September 1) Malignant Carcinoid Syndrome. eMedicine [On-line information]. Available online at http://emedicine.medscape.com/article/282515-overview. Accessed December 2010.
(© 1995-2010). Unit Code 84395: Serotonin, Serum. Mayo Clinic, Mayo Medical Laboratories [On-line information]. Available online at http://www.mayomedicallaboratories.com/test-catalog/Overview/84395. Accessed December 2010.
Frank, E. (Updated 2010 January). Carcinoid Tumors. ARUP Consult [On-line information]. Available online at http://www.arupconsult.com/Topics/CarcinoidTumors.html?client_ID=LTD. Accessed December 2010.
(Revised 2010 July 13). Detailed Guide, Gastrointestinal Carcinoid Tumors. American Cancer Society [On-line information]. Available online at http://www.cancer.org/Cancer/GastrointestinalCarcinoidTumor/DetailedGuide/index. Accessed December 2010.
(Revised 2010 August 17). Detailed Guide, Lung Carcinoid Tumors. American Cancer Society [On-line information]. Available online at http://www.cancer.org/Cancer/LungCarcinoidTumor/DetailedGuide/lung-carcinoid-tumor-what-is-lung-carcinoid-tumor. Accessed December 2010.
Vorvick, L. (Updated 2009 March 14). Serum serotonin level. MedlinePlus Medical Encyclopedia [On-line information]. Available online at http://www.nlm.nih.gov/medlineplus/ency/article/003562.htm. Accessed December 2010.
Pagana, K. D. & Pagana, T. J. (© 2011). Mosby’s Diagnostic and Laboratory Test Reference 10th Edition: Mosby, Inc., Saint Louis, MO. Pp 871-873.
Henry’s Clinical Diagnosis and Management by Laboratory Methods. 21st ed. McPherson R, Pincus M, eds. Philadelphia, PA: Saunders Elsevier: 2007, Pg 284.
Tebbi, C. (2015 June 17, Updated). Carcinoid Tumor. Medscape Drugs & Diseases. [On-line information]. Available online at http://emedicine.medscape.com/article/986050-overview#showall. Accessed 11/15/15.
Chen, Y. (2014 August 31, Updated). Carcinoid syndrome. MedlinePlus Medical Encyclopedia [On-line information]. Available online at http://www.nlm.nih.gov/medlineplus/ency/article/000347.htm. Accessed 11/15/15.
(© 1995–2015). Serotonin, Blood. Mayo Clinic Mayo Medical Laboratories [On-line information]. Available online at http://www.mayomedicallaboratories.com/test-catalog/Clinical+and+Interpretive/84373. Accessed 11/15/15.
Frank, E. et. al. (2015 February, Updated). Carcinoid Tumors. ARUP Consult [On-line information]. Available online at http://www.arupconsult.com/Topics/CarcinoidTumors.html?client_ID=LTD. Accessed 11/15/15.
Vorvick, L. (2013 January 22, Updated). Serum serotonin level. MedlinePlus Medical Encyclopedia [On-line information]. Available online at http://www.nlm.nih.gov/medlineplus/ency/article/003562.htm. Accessed 11/15/15.
Mayo Clinic Staff (© 1995-2015). Serotonin Syndrome. Mayo Clinic [On-line information]. Available online at http://www.mayoclinic.org/diseases-conditions/serotonin-syndrome/basics/definition/CON-20028946. Accessed 11/15/15.
Pagana, K. D., Pagana, T. J., and Pagana, T. N. (© 2015). Mosby’s Diagnostic & Laboratory Test Reference 12th Edition: Mosby, Inc., Saint Louis, MO. Pp 823-824.
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