A 22-year-old woman is evaluated for hypoglycemia suspected to be self-induced. Plasma insulin is markedly elevated, C-peptide is undetectable, and proinsulin is absent. Urine sulfonylurea screen is negative. These findings indicate which mechanism?

• A Insulinoma secreting mature insulin autonomously
• B Sulfonylurea-stimulated endogenous insulin release
• C Exogenous insulin administration ✓
• D Autoimmune insulin syndrome with binding antibodies

Explanation

B-peptide is cleaved equimolarly from proinsulin during endogenous beta-cell secretion, so any endogenous insulin rise carries a parallel B-peptide rise. Elevated insulin with undetectable B-peptide therefore proves exogenous insulin injection. An insulinoma or sulfonylurea effect would show elevated insulin WITH elevated B-peptide and proinsulin, making option A the strongest distractor and killed by the absent proinsulin. Autoimmune insulin syndrome typically shows high insulin with variable but usually detectable B-peptide.

Reference: Williams Textbook of Endocrinology, 14th ed.

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