Relapse of Medically Treated Insulinoma Following the Development of Thyrotoxicosis: An Assessment of Potential Mechanisms

Increased thyroid hormone concentrations have been reported to have disparate effects on insulin sensitivity in man. We describe a 72-year-old lady who initially presented with episodic hypoglycemia secondary to an insulinoma that was controlled by diazoxide. She re-presented 12 months later with a recurrence of the hypoglycemia following the development of thyrotoxicosis. The diazoxide treatment was maintained and propranolol was introduced, which prevented further episodes of hypoglycemia. This appeared to be due to a direct effect of propranolol on endogenous insulin secretion, while whole body insulin sensitivity remained unchanged as assessed using the hyperinsulinemiceuglycemic clamp technique. She was later rendered biochemically euthyroid with a combination of blocking carbimazole therapy and thyroxine replacement, and this was associated with a marked decrease in insulin sensitivity. Thus, the principal effect of thyroid hormone excess in this patient was an increase in insulin sensitivity that led to the clinical relapse of the insulinoma.