C-Peptide is a byproduct of insulin production, usually by the pancreas. The level of C-peptide can be used as a gauge of how much insulin is being produced in the body. The amount of C-peptide in the blood can indicate the presence or absence of disease. Abnormally low amounts of C-peptide in the blood suggest that insulin production is too low or absent because of type I diabetes, also known as juvenile or insulin-dependent diabetes. Abnormally high amounts of C-peptide warn of the possible presence of insulinoma tumors, which secrete insulin.