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Difficulty: MediumEndocrine and Metabolic Mechanisms

A 38-year-old patient is admitted to the diagnostic evaluation unit for recurrent episodes of diaphoresis, palpitations, and confusion that resolve rapidly with carbohydrate intake. A supervised 72-hour fasting test is initiated, and comprehensive laboratory studies are drawn when the patient develops symptomatic hypoglycemia (capillary blood glucose 42 mg/dL). Match each distinct laboratory profile below with its most likely underlying pathophysiologic mechanism.

  • Serum insulin elevated, C-peptide elevated, proinsulin elevated, negative oral secretagogue screenAutonomous neuroendocrine vesicular exocytosis
  • Serum insulin elevated, C-peptide suppressed, proinsulin suppressed, negative oral secretagogue screenDirect administration of a bioengineered active hormone
  • Serum insulin elevated, C-peptide elevated, proinsulin normal, positive oral secretagogue screenPharmacologic closure of ATP-sensitive potassium channels
  • Serum insulin suppressed, C-peptide suppressed, elevated IGF-II, low beta-hydroxybutyrateParaneoplastic activation of peripheral insulin receptors

Answer

Profile L1 matches autonomous exocytosis (insulinoma). Profile L2 matches direct hormone administration (exogenous insulin). Profile L3 matches K-ATP channel closure (sulfonylurea). Profile L4 matches paraneoplastic receptor activation (non-islet cell tumor hypoglycemia).
Each laboratory profile uniquely maps to a pathophysiologic mechanism of hypoglycemia. Elevated insulin with high C-peptide and a negative drug screen points to an insulinoma (autonomous neuroendocrine secretion). High insulin with low C-peptide indicates exogenous insulin injection, as commercial preparations lack the cleavage byproduct. High insulin and C-peptide with a positive drug screen confirms sulfonylurea use, which pharmacologically closes K-ATP channels. Low insulin and C-peptide with high IGF-II indicates a paraneoplastic syndrome where the tumor secretes a peptide that activates peripheral insulin receptors.

Step-by-Step Solution

1
Analyze the relationship between serum insulin and C-peptide to differentiate endogenous from exogenous hyperinsulinemia.
Elevated insulin with suppressed C-peptide identifies exogenous administration, while concurrent elevation of both points to an endogenous source.
Endogenous insulin is secreted in a 1:1 equimolar ratio with C-peptide, whereas pharmaceutical insulin preparations do not contain C-peptide.
2
Evaluate the oral secretagogue screen in profiles with endogenous hyperinsulinemia.
A positive screen identifies pharmacologic stimulation of beta cells, whereas a negative screen points to autonomous tumor secretion.
Sulfonylureas and meglitinides artificially stimulate endogenous insulin release by closing K-ATP channels, mimicking an insulinoma.
3
Identify non-insulin-mediated mechanisms in profiles with suppressed insulin and C-peptide.
Hypoglycemia with suppressed beta-cell function and elevated IGF-II indicates a paraneoplastic syndrome.
Tumor-secreted IGF-II binds to insulin receptors, driving glucose into tissues while appropriately suppressing endogenous pancreatic beta-cell activity via negative feedback.

Key Concept

Pathophysiologic mechanisms and diagnostic laboratory profiles of fasting hypoglycemia
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Topics

Endocrine and Metabolic Mechanisms

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