What relationship does the serotonin level have in the regulation of carcinoid tumors?

Serotonin is a chemical product produced by some carcinoids in various amounts. It does not regulate the tumors. When in large amounts, it causes symptoms but in any amount, it is a useful marker for the presence of each tumor. 5HTP is the chemical precursor (for the substance) from which serotonin is made and is also a useful marker sometimes.