Propranolol and hyperthyroidism: Serum free fatty acids and glucose-induced insulin release in nondiabetic thyrotoxic patients during treatment to clinical compensation

Abstract

Glucose-induced insulin secretion was studied in 10 hyperthyroid patients, without personal or familial diabetic background, treated with increasing weekly doses of propranolol until clinical compensation was obtained. Intravenous glucose tolerance tests (25g) with concomitant determination of serum glucose, insulin and FFA were done before treatment, at 240 mg/day of propranolol and after clinical compensation. Patients were divided into two groups according to laboratory data and propranolol dosage needed for clinical compensation (group A: 240 mg/day; group B: 320–400 mg/day). Fasting serum insulin, glucose disappearance rate, and estimates of total insulin secretion after intravenous glucose did not change significantly during propranolol therapy and were within the normal range. Fasting FFA of untreated patients were significantly higher than control values ( p<0.001), but a significant decrease was already seen at 240 mg/day of propranolol, even before clinical compensation. There was a marked difference in the insulin secretion pattern of thyrotoxic patients as compared to controls. Serum insulin and insulin:glucose ratios increased promptly, and at 5 min after glucose reached significantly higher levels than in normal subjects, before treatment as well as after clinical compensation with the propranolol therapy. Both the increased levels of FFA and of T 3 could be involved in this pattern of the insulin response of nondiabetic thyrotoxic patients, since the secretion of insulin during the first 10 min after intravenous glucose was directly correlated to fasting serum FFA before propranolol, and serum T 3 was directly correlated with total insulin response after clinical compensation. Furthermore the comprison of the results obtained in group A and group B patients raises the possibility that an increased beta-cell responsiveness to beta-adrenergic stimuli might also be involved in this pattern of insulin secretion.