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Subject: C-Peptide

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  • Human C-peptide is a 31-amino-acid chain with a molecular mass of approximately 3,020 Da. Metabolically inert, it originates in the pancreatic B cells as a by-product of the enzymatic cleavage of proinsulin to insulin. In this process, insulin and C-peptide are split from the prohormone and secreted into the portal circulation in equimolar concentrations. Within limits, C-peptide levels can serve as a valuable index to insulin secretion. Therefore, low C-peptide levels are to be expected where insulin secretion is diminished, as in insulin-dependent diabetes, or suppressed, as a normal response to exogenous insulin, whereas elevated C-peptide levels may result from the increased B-cell activity observed in insulinomas.

  • Normal range: 0.9–7.1 ng/mL.


  • For estimating insulin levels in the presence of antibodies to exogenous insulin

  • Diagnosis of factitious hypoglycemia due to surreptitious administration of insulin in which high serum insulin levels occur with low C-peptide levels

  • Evaluation of insulinoma

  • Monitoring pancreatic and islet cell transplant function


Increased In

  • Insulinoma

  • Type 2 DM

Decreased In

  • Exogenous insulin administration (e.g., factitious hypoglycemia)

  • Type 1 DM


  • C-peptide serum levels correlate with insulin levels in blood, except in islet cell tumors and possibly in obese patients.