Propranolol-induced increments in total and free serum thyroxine in patients with essential hypertension

The effect of propranolol on serum levels of total and free thyroxine, total and free triiodothyronine, and thyroid-stimulating hormone (TSH) was investigated in 15 clinical euthyroid patients with essential hypertension. Oral propranolol in dosages from 80 to 480 mg/day for a 30-day period induced an average increase in total serum thyroxine of 15.5 +/- 2.2% (mean +/- SEM, 2 p = 0.00002) and in free thyroxine of 17.7 +/- 3.5% (2 p = 0.00009). The oral dose of propranolol correlated positively with serum propranolol (r = 0.70, 2 p = 0.007). No significant correlation between serum propranolol and changes in serum thyroxine could be demonstrated. Total and free triiodothyronine, as well as TSH, remained unchanged during propranolol treatment. The most likely explanation is a propranolol-induced decreased degradation of thyroxine. A practical consequence is that in patients with an uncertain clinical picture and slightly elevated serum thyroxine, propranolol intake should be considered.