Glucose intolerance occurs in about 30% of patients with pheochromocytoma, but overt diabetes mellitus is uncommon.182 Mechanisms responsible for glucose intolerance include suppression of insulin by α-adrenergic receptor stimulation of β cells; an increase in insulin resistance, probably related to elevated plasma free fatty acid levels; and increased hepatic glucose output as a result of β-adrenergic stimulation of hepatocytes. α-Adrenergic receptor blockade improves glucose tolerance and insulin secretion.