C-peptide (connecting peptide) is a 31-amino-acid byproduct of endogenous insulin biosynthesis, cleaved from proinsulin in equimolar amounts to insulin within pancreatic beta cells. Because it is not subject to first-pass hepatic extraction and has a longer half-life (~30 minutes) than insulin (~5 minutes), it serves as a more stable and reliable marker of endogenous insulin secretion. Measurement of C-peptide allows clinicians to distinguish endogenous from exogenous insulin production and to assess residual beta-cell function. Proinsulin is synthesized in pancreatic beta cells and cleaved by prohormone convertases into equimolar quantities of insulin and C-peptide, both of which are co-secreted into the portal circulation. Unlike insulin, C-peptide undergoes minimal hepatic clearance and is primarily eliminated by the kidneys, resulting in peripheral C-peptide concentrations approximately 5–10 times higher than insulin. C-peptide itself has emerging biological activity, including potential roles in microvascular function and cellular signaling, though its primary clinical utility remains as a surrogate marker of beta-cell secretory capacity.
C-peptide is a small protein released by your pancreas in equal amounts alongside insulin, making it a useful indicator of how much insulin your body is producing on its own. Because it stays in the bloodstream longer than insulin, it gives a clearer picture of your pancreas's activity. Doctors use this test to understand whether low blood sugar is caused by your own pancreas making too much insulin or by taking too much insulin medication. It also helps distinguish between different types of diabetes and monitor how well the insulin-producing cells in your pancreas are working over time. Results are always interpreted alongside other tests and your overall health history.
When elevated: Elevated C-peptide suggests increased endogenous insulin secretion, which may reflect insulin resistance (as in obesity or type 2 diabetes), insulinoma or other insulin-secreting tumors, use of insulin secretagogues (e.g., sulfonylureas), or early compensatory hyperinsulinemia. Persistently high levels in the context of hypoglycemia raise concern for an insulin-secreting neoplasm. When low: Low or undetectable C-peptide indicates diminished or absent endogenous insulin secretion, consistent with autoimmune destruction of beta cells (type 1 diabetes), advanced beta-cell exhaustion in longstanding type 2 diabetes, or post-pancreatectomy states. Very low levels in a hypoglycemic patient receiving insulin suggest exogenous insulin administration rather than an endogenous source.
C-peptide is not a routine performance or training-specific biomarker. However, elevated fasting C-peptide may indicate insulin resistance, which can impair metabolic efficiency, recovery, and long-term metabolic health in athletes. Tracking C-peptide alongside glucose and insulin can help athletes and active individuals identify subclinical metabolic dysfunction early, supporting nutrition and training adjustments to preserve insulin sensitivity.
Turnaround Time
3 days (up to 14 days)
Fasting Required
No
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Versus the typical direct-to-consumer retail price for this test (illustrative — consumer prices vary by provider and region).
$16.00
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$33.00
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