C-peptide is a peptide which is made when proinsulin is split into insulin and C-peptide. They split when released from the pancreas and is released into the blood - one C-peptide for each insulin molecule. C-peptide is the abbreviation for connecting peptide.

C-peptide functions in repair of the muscular layer of the arteries.

Newly diagnosed diabetes patient often get their C-peptide levels measured, to find if they are type 1 diabetes or type 2 diabetes. The pancreas of patients with type 1 diabetes is unable to produce insulin and they will therefore usually have a decreased level of C-peptide, while C-peptide levels in type 2 patients is normal or higher than normal. Measuring C-peptide in patients injecting insulin can help to determine how much of their own natural insulin these patients are still producing.