C-peptide is equimolar to insulin in secretion from beta cells. In a patient with insulin-induced hypoglycaemia (factitious hypoglycaemia), what is the expected C-peptide level?

• A High — because endogenous insulin secretion is stimulated by hypoglycaemia
• B Normal — C-peptide production is independent of insulin secretion
• C Low — because exogenous insulin suppresses endogenous insulin/C-peptide ✓
• D Undetectable — because C-peptide is destroyed in the liver

Explanation

Proinsulin is cleaved in the beta-cell secretory granule into equimolar amounts of insulin and C-peptide, which are co-secreted. Exogenous insulin (injected) suppresses beta-cell activity via hypoglycaemia (which would normally stimulate secretion but in the setting of already-high insulin the KATP channels are suppressed) — actually, the key: exogenous insulin does not contain C-peptide, and the high insulin level suppresses residual endogenous insulin secretion via feedback. Thus in factitious hypoglycaemia: high insulin level + low/undetectable C-peptide. In insulinoma: both insulin and C-peptide are elevated.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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Written and medically reviewed by the StethoPrep medical team.