The carcinoid tumor must have metastasized to the lungs to cause damage to left-sided heart valves.

Carcinoid tumors are well known to cause damage to right-sided heart valves. Tricuspid stenosis or regurgitation can occur and has a typical thickened, immobile appearance of the tricuspid valve leaflets due to fibrosis. The toxic substance secreted by the tumor is likely serotonin, although research is ongoing. Serotonin is easily metabolized by the liver and carcinoid tumors frequently originate in the GI tract. This results in the toxic serotonin getting metabolized before reaching the right heart not causing any cardiac abnormality, unless the tumor metastasizes to the liver.

Carcinoid tumors very rarely affect left-sided heart valves. The serotonin is also metabolized in the lung parenchyma. In order for the serotonin to reach the left heart the tumor must metastasize to the lungs bypassing this filtration.