The Influence of β-Adrenoceptor Blocking Agents on Plasma Thyroxine and Triiodothyronine

Abstract

The effect of the .beta.-adrenoceptor blocking agent propranolol on plasma levels of thyroxine and triiodothyronine was studied in 11 hyperthyroid patients and 6 hypothyroid patients on L-thyroxine substitution therapy. In all patients a decrease in plasma triiodothyronine was found. The per cent decrease of plasma triiodothyronine was the same in both groups. Plasma levels of thyroxine and thyrotropin increased in the hypothyroid patients, but remained constant in the hyperthyroid subjects. The decrease in plasma triiodothyronine during propranolol medication is probably caused by an inhibition of the peripheral conversion of thyroxine into triiodothyronine.