Plasma C peptide and diagnosis of factitious hyperinsulinism. Study of an insulin dependent diabetic patient with 'spontaneous' hypoglycemia

Factitious hyperinsulinism was suspected in a patient with an 11 yr history of insulin dependent, ketosis prone diabetes mellitus who began to experience repetitive episodes of 'spontaneous' hypoglycemia. Insulin mediation of the hypoglycemia was confirmed by documenting that urinary insulin and total extractable insulin in plasma increased during periods of hypoglycemia. Failure to detect significant amounts of human C peptide by radioimmunoassay during periods of hypoglycemia or in response to stimulation with glucagon, leucine, or tolbutamide indicated that the insulin was not endogenous. The diagnosis of factitious hyperinsulinism was ultimately proved by the finding of radioactivity in the patient's urine after ¹³¹I Hippuran was added to a vial of insulin found in the patient's room.