Insulin is secreted by the pancreatic beta-cells as a prohormone composed of fragments: C-peptide and insulin. The C-peptide fraction is cleaved off the prohormone. The insulin fraction becomes active. C-peptide is inactive but provides structure to the prohormone and has a much longer half-life. Both of these hormones can be quantitated in blood.
Insulin levels are not measured to diagnose or monitor diabetes, but can give information about a patient and is an important assay in low blood sugar (hypoglycemia). C-peptide is also measured in evaluating hypoglycemia and is used to distinguish between endogenous and exogenous insulin; it would be present in circulation in endogenous insulin secretion. In addition, it is also often used to monitor pancreatic surgery and transplant because of its longer half-life.