C-peptide
C-peptide is released 1:1 with insulin when the pancreas makes it, so it measures the patient's own insulin production — the cleanest way to distinguish type-1 from type-2 diabetes and to detect surreptitious insulin use.
What it measures
Proinsulin in the pancreatic beta cell is cleaved into insulin and connecting peptide (C-peptide) in 1:1 stoichiometry. Injected insulin contains no C-peptide, so a C-peptide level on a sample taken with glucose reflects only endogenous (the patient's own) pancreatic output. C-peptide has a longer half-life than insulin (~30 min vs ~5 min) and isn't extracted by the liver, so it's a stabler estimate of beta-cell function than insulin itself.
What a high value can mean
- Type 2 diabetes (early/mid) — insulin resistance drives compensatory hypersecretion.
- Insulinoma — pancreatic beta-cell tumour; very high C-peptide despite hypoglycaemia.
- Sulfonylurea overuse — these drugs stimulate endogenous insulin; differential vs insulinoma.
- Cushing's syndrome, acromegaly — counter-regulatory hormone excess drives insulin secretion.
What a low value can mean
- Type 1 diabetes — autoimmune beta-cell destruction; classic finding is high glucose + low C-peptide.
- Long-standing type 2 diabetes — beta-cell exhaustion after years of compensation.
- Pancreatectomy, severe chronic pancreatitis — anatomic loss of beta cells.
- Surreptitious insulin use — exogenous insulin causes hypoglycaemia with suppressed C-peptide.
When to discuss with a doctor
C-peptide is ordered when the type of diabetes is unclear (adult-onset that doesn't behave like type-2, lean patient on insulin who 'might not need it'), or to investigate hypoglycaemia (insulinoma vs factitious insulin use). It's not part of routine diabetes monitoring — HbA1c handles that. Mediora.AI shows C-peptide against glucose so the pattern (preserved vs absent insulin reserve) is visible; the diabetes-type decision is endocrinology.