MEDizzy - A 40-year-old physician’s assistant has had episodic

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Combined Endocrinology III

A 40-year-old physician’s assistant has had episodic confusion, diaphoresis, and palpitations for the past 4 weeks. She has had several nightmares and three syncopal episodes. Fasting hypoglycemia with inappropriately elevated plasma insulin concentration is documented in the hospital. Plasma C-peptide concentration also is increased. Her physician should

ExplanationC. Because factitious hypoglycemia resulting from insulin injection or sulfonylurea ingestion is common, the finding of hyperinsulinemia associated with a low blood sugar concentration can no longer be considered diagnostic of an islet cell tumor (insulinoma). Suspicion of factitious disease should be especially high in medical personnel and in the relatives of diabetics. The alpha and beta subunits of insulin are cleaved from proinsulin in the beta cell and are released in equimolar amounts with the connecting (C) peptide; elevation of plasma C-peptide levels signifies endogenous hyperinsulinemia, because exogenous insulin administration suppresses beta-cell function. Therefore, the triad of fasting hypoglycemia, hyperinsulinemia, and elevated plasma C-peptide levels is consistent with either endogenous hyperinsulinemia or the ingestion of a sulfonylurea; documentation of the latter in urine or plasma would be diagnostic. Proinsulin usually is released into the circulation in small quantities. However, in patients with insulinoma, proinsulin concentration frequently exceeds 20% of total insulin; ingestion of a sulfonylurea, by contrast, does not cause a disproportionate elevation of plasma proinsulin levels. Insulin antibody measurements in this case would not be expected to be helpful; antibodies may not develop for several months after the start of insulin injections, and the high C-peptide levels essentially rule out an exogenous source of insulin. However, in some circumstances antibodies to specific species of insulin can be identified and hence establish that exogenous insulin has been taken. Attempts to localize an islet cell tumor by radiologic means should be done only after factitious types of hypoglycemia have been excluded.