Hyperinsulinism after removal of a pheochromocytoma

The finding of hypoglycemia after the surgical removal of a pheochromocytoma in two patients in a previous study led to monitoring of the serum glucose and plasma C-peptide levels in two other patients with a pheochromocytoma and one with unilateral adrenocortical hyperplasia. In the two patients with a pheochromocytoma endogenous insulin secretion, as measured by a C-peptide assay, was suppressed before removal of the tumours and resumed immediately after removal. The serum glucose levels decreased in these patients, but sufficient intravenous administration of glucose prevented postoperative hypoglycemia. In the patient with adrenocortical hyperplasia the plasma C-peptide level was not decreased before tumour removal, nor did it increase abruptly following removal. It therefore seems likely that the rapid fall in the serum glucose level following removal of a pheochromocytoma is caused by prompt resumption of beta-cell activity, with rebound hyperinsulinism.