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C peptideA biologically inactive moiety of proinsulin produced endogenously in the pancreas and stored in secretory granules in a 1:1 ratio with insulin; unlike factitious hypoglycemia, which is induced by insulin of exogenous origin, an increased C-peptide (= 0.2 nmol/L) as well as increased insulin (= 42 pmol/L) is characteristic of insulinoma. C peptide quantification is used to detect fictitious insulin injection and diagnose insulin-secreting tumours in diabetics, where > 7 ng/ml of C peptide after induced hypoglycemia supports the diagnosis of insulinoma.
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