Insulin is a hormone produced by the beta cells of the pancreas. It is essential for allowing glucose to enter the cells. Insulin is produced in the form of a larger precursor molecule called proinsulin, part of which (C peptide) must be removed to produce active insulin. Insulin drug preparations used to treat diabetes do not contain this inactive fragment. Results are expressed in nanomoles per litre of C peptide (nmol/L) and correspond to insulin production by the patient’s pancreas (endogenous production) even in patients treated with exogenous insulin (insulin drug preparation).
High C peptide levels generally indicate strong endogenous insulin production by the pancreas. This can be normal in response to high or abnormal blood glucose levels to indicate insulin resistance by the tissues (obesity, type 2 diabetes).
High C peptide levels can also be seen in insulinoma (benign pancreatic tumour) and other conditions (low potassium, overactive adrenal glands, kidney failure). In individuals with insulinoma, falling C peptide levels may indicate that they are responding well to treatment. Low C peptide levels are typical of type 1 diabetes and are seen in patients being treated with exogenous insulin.