The influence of beta-adrenoceptor blocking agents on plasma thyroxine and triiodothyronine

The effect of the beta-adrenoceptor blocking agent propranolol on plasma levels of thyroxine and triiodothyronine was studied in eleven hyperthyroid patients and in six 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 most likely caused by an inhibition of the peripheral conversion of thyroxine into triiodothyronine.