Explain how elevated insulin level and suppressed c-peptide level strongly supports exogenous insulin administration, as opposed to overproduction of insulin in such cases of endogenous insulin overproduction as insulinoma or treatment with sulphonylurea

Equimolar amount of insulin and C peptide are usually released into the body system circulation under normal circumstances. A large fraction of circulating insulin is typically rapidly remove by the liver while the liver clear only a negligible amount of C peptide. C peptide is primarily cleared by the kidney at a lower metabolic clearance rate.
Normally the, C peptide level increases when the pancreatic secretion of insulin increases and vice versa. Therefore, exogenous administration of insulin results in an increase in insulin level without a corresponding increase in C peptide level. On the other hand, both insulin and C peptide levels are elevated in patients with insulinoma.