C-peptide or connecting peptide is a marker of insulin production by the body. Normal values vary from laboratory to laboratory and its level is read along with blood sugar levels. Fasting levels range between 0.5–3.0 ng/ml while post-prandial levels may range from 1.2 to 3.5 ng/ml.
A high level of C-peptide level indicates high level of insulin production. This may be in response to raised blood sugar level, insulin resistance, certain tumors or other disease states like kidney disorder, use of medicines etc. You would need to discuss with the treating doctor who will be best placed to advise you.