Pharmacokinetics of propranolol used for suppressing tachycardia in hyperthyroidism.

The aim of this study was to investigate the pharmacokinetics of propranolol in patients with hyperthyroidism, and its effect on heart rate (HR) using 24 hour-ECG monitoring. Subjects included 9 patients with hyperthyroidism (group H) and 9 euthyroid patients with hypertension selected as the control group (group C). All patients took propranolol at a dosage of 10mg, 3 times a day for 7 days. Plasma propranolol concentrations were measured on the 7th day of its administration by the HPLC method. Twenty-four hour-ECG monitoring was recorded twice before and after treatment with propranolol. The mean reduction in the minimum HR was 10.7±3.6 (11.9±3.6%) in group H and 5.0±1.6 (9.5±3.0%)bpm in group C. That in the average HR was 12.2±3.7 (11.3±3.1%) in group H and 9.2±2.2bpm (13.7±3.2%) in group C. That in the maximum HR was 16.9±5.2 (10.9±3.1%) in group H and 24.4±2.4bpm (21.9±2.1%) in group C. The peak concentration of propranolol in plasma was 10.8±2.4ng/ml in group H and 55.3±14.3ng/ml in group C (p<0.01). There was a linear correlation between the reduction in the hourly maximum heart rate and the peak plasma propranolol concentration in group H. Cardiovascular manifestations in hyperthyroid patients were related to sympathetic hypersensitivity and their improvement was not fully obtained by small doses of propranolol.