C-Peptide

Endogenous insulin production marker — cleaner than fasting insulin for measuring what your pancreas is actually making.

What C-Peptide Measures

C-peptide is cleaved from proinsulin in a 1:1 ratio with insulin, but unlike insulin it is not cleared by the liver on first-pass. This makes it a more stable and honest reflection of pancreatic beta-cell output. In patients on exogenous insulin or GLP-1s, C-peptide shows endogenous production separately from exogenous hormone.

It is the gold standard for distinguishing T1D (low C-peptide) from T2D (normal-to-high C-peptide, reflecting insulin resistance).

What Affects This Biomarker

C-peptide is influenced by: beta-cell mass and function, insulin resistance (higher output compensating), post-prandial stimulation (rises after meals), renal function (accumulates in CKD), and GLP-1 therapy (raises glucose-dependent insulin secretion during meals, usually lowers fasting values over time via improved insulin sensitivity).

In the Context of Peptide Protocols

Order when fasting insulin is equivocal, to confirm endogenous production on GLP-1 therapy, or to distinguish early autoimmune diabetes from insulin resistance. Not a routine repeat — baseline and then again if clinical picture changes.

Peptides That Commonly Move C-Peptide

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