The connecting peptide, or C-peptide, is a short 31-amino-acid polypeptide that connects insulin’s A-chain to its B-chain in the proinsulin molecule.
C peptide = Connecting peptide.
C peptide is increased with insulinomas and sulfonylureas and decreased with exogenous insulin.
“C-peptide is produced in equal amounts to insulin and is the best measure of endogenous insulin secretion in patients with diabetes. Measurement of insulin secretion using C-peptide can be helpful in clinical practice: differences in insulin secretion are fundamental to the different treatment requirements of Type 1 and Type 2 diabetes. ”
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3748788/