Carcinoid syndrome is the pattern of symptoms which are exhibited by people with carcinoid tumors. These tumors can occur in the small intestine, colon, bronchial tubes, and appendix.

Carcinoid tumors secrete excessive amounts of the hormone serotonin as well as other chemicals that cause the blood vessels to dilate (widen). These tumors can also cause diarrhea and wheezing (asthma).

There are wide variations in the chemicals secreted by these tumors, and the symptoms depend on the mix of chemicals made by the particular tumor.

Normally, an amino acid, called tryptophan, is used by the body to manufacture niacin and certain proteins, but in carcinoid syndrome, it gets diverted to form serotonin. Most serotonin is converted by the body to 5-hydroxy indole acetic acid (5-HIAA).

The most bothersome symptoms include bright red facial flushing, diarrhea (which may be explosive and severe) and occasionally wheezing. A specific type of heart valve damage can occur, as well as other cardiac problems.

In children, carcinoid tumors usually occur in the appendix, and removal of the appendix usually results in a complete cure.