C - Peptide
When insulin is released into the blood to help with the transport of glucose, equal amounts of C-peptide also are released. Although C-peptide does not have an effect on blood sugar levels, C-peptide is a useful marker of insulin production as it remains in the blood longer than insulin.
What might a high result mean?
High concentrations of C-peptide within the bloodstream generally indicate high concentrations of insulin. This may be due to excessive insulin production by the body, a response to high concentrations of blood glucose caused by glucose intake or insulin resistance (when the body's cells do not respond normally to insulin and so the body makes more insulin in an attempt to compensate).