Propranolol and thyroid hormone metabolism.

Propranolol decreases plasma T3 and increases plasma rT3 in a dose-dependent manner due to a decreased production rate of T3 and a decreased metabolic clearance rate of rT3, respectively, caused by inhibition of the conversion of T4 into T3 and of rT3 into 3,3'-T2. This inhibition of 5'-deiodination is not secondary to inhibition of thyroid hormone transport across the plasma membrane. Propranolol and its major metabolite, 4-hydroxypropranolol, are not directly responsible for these effects, but an unidentified metabolite of propranolol might be involved. beta-blockers ameliorate clinical symptoms and signs of thyrotoxicosis independent of the decrease of plasma 13, that is confined to beta-blockers with membrane-stabilizing activity, such as propranolol and alprenolol. The decrease of plasma T3, however, appears responsible for some of the metabolic responses to beta-blockers.