C-peptide

C-peptide is the leftover connector snipped out when proinsulin becomes insulin — and although once dismissed as biological packing material, it has turned out to be a remarkably useful messenger to clinicians. Because exactly one molecule of C-peptide is released for every molecule of insulin, measuring it reveals how much insulin a person's own pancreas is making.

This matters because injected insulin contains no C-peptide. In someone taking insulin, a blood insulin level cannot distinguish injected drug from self-made hormone, but a C-peptide level reflects only the patient's own beta-cell output. It is therefore a standard test to gauge endogenous insulin secretion.

Clinically, low or undetectable C-peptide points to severe beta-cell loss, as in type 1 diabetes, whereas normal or high levels support type 2 diabetes or insulin resistance. A high C-peptide together with low blood sugar can also flag an insulin-secreting tumor or factitious insulin abuse — the latter showing high insulin but low C-peptide.