C-Peptide is a 31-amino acid peptide that is produced in equal amounts with insulin during the processing of proinsulin in pancreatic beta cells. This test measures C-peptide levels in the blood to evaluate endogenous insulin production and pancreatic beta-cell function. C-peptide is particularly valuable for distinguishing between endogenous and exogenous insulin, as it is only produced when the pancreas makes insulin naturally. Elevated C-peptide levels in the presence of hypoglycemia suggest insulinoma or other insulin-secreting tumors, while low C-peptide levels with high insulin suggest factitious hypoglycemia from exogenous insulin use. The test is also useful for evaluating residual beta-cell function in type 1 diabetics and distinguishing between different types of diabetes. C-peptide measurements help guide treatment decisions and assess the success of pancreatic transplantation or islet cell therapy.