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What Causes Carcinoid Syndrome?

Carcinoid syndrome occurs when carcinoid tumors overproduce hormones and other substances that normally circulate throughout your body. One of the most important substances that is overproduced is serotonin—one of the body's natural chemical messengers. When excess serotonin reaches tissues in the gastrointestinal tract, the lungs, or the skin, it causes the symptoms of carcinoid syndrome.

Carcinoid tumors often do not produce noticeable symptoms until they spread to the liver. That's because most of the blood circulation from the gastrointestinal tract must pass through the liver before it reaches the rest of the body. The liver has strong enzymes that break down and neutralize most of the excess serotonin and other substances produced by the carcinoid tumors, preventing them from reaching tissues where they can cause symptoms. When carcinoid tumors metastasize to the liver, the substances they overproduce can more easily reach your bloodstream, and reach tissues where they can cause symptoms.

Serotonin breaks down into smaller parts, called metabolites. The most important serotonin metabolite is 5-HIAA (5-hydroxyindoleacetic acid). The 5-HIAA test measures the amount of this metabolite in your blood, and can therefore be used to estimate the extent of your disease. Chromogranin-A (CgA) is a protein found in carcinoid cells that may be secreted into the blood. CgA can also be measured to check the extent of carcinoid syndrome.

Click here to learn more about tests used to diagnose and monitor carcinoid syndrome.
Chromogranin A test
5-HIAA urine test

Learn more about the symptoms of carcinoid syndrome