The influence of propranolol on the extrathyroidal metabolism of 3,3',5'-triiodothyronine (reverse T3).

The effect of propranolol 80 mg daily on the metabolism of 3,3',5'-triiodothyronine (reverse T3, rT3), 3,3',5'-triiodothyronine (T3) and thyroxine (T4) was studied by means of a non compartmental kinetic method in seven females with severe pretreatment hypothyroidism. The patients were maintained euthyroid on a constant L-T4 replacement therapy. Serum rT3 levels increased significantly during propranolol (p less than 0.02). This increase was explained by a decrease in metabolic clearance rate (MCR) (p less than 0.02), since the conversion rate from T4 and the distribution volume of rT3 were unchanged. By contrast the decreased serum levels of T3 were due to a significant decreased conversion from T4 (p less than 0.02) in spite of a decreased MCR. The results are compatible with the assumption of two different monodeiodinating enzymes, a 5-deiodinase responsible for the diodination of T4 to rT3 and a 5'-deiodinase responsible for the deiodination of T4 to T3.