Carcinoid tumor cells can produce a variety of biologically active substances, the most important of which are 5-hydroxytryptamine, bradykinin, histamine and prostaglandins. 5-hydroxytryptamine is derived from tryptophan in food. After the development of carcinoid tumors, 60% of the tryptophan in food is converted into 5-hydroxytryptamine in carcinoid tumor cells, so there is a significant increase of 5-hydroxytryptamine in the blood of carcinoid tumor patients. Bradykinin is another relatively major bioactive substance causing clinical manifestations. In carcinoid tissues, there is a large amount of vasodilator, a protein hydrolase that acts on kininogen to produce lysergic acid bradykinin (tryptokinin), which is converted to bradykinin by the action of aminopeptidase. Bradykinin causes skin flushing and skin temperature is not high, which is the cold type. 5-hydroxytryptophan causes skin flushing and skin warmth, which is the hot type. If liver metastasis is already present, on the one hand, because of the excessive production of 5-hydroxytryptamine, on the other hand, it can directly enter the hepatic vein and enter the body circulation. Furthermore, the liver has a reduced clearance function due to the extensive carcinoid invasion. Therefore, carcinoid syndrome can occur after the presence of liver metastases.