The extrathyroidal effect of D,L-propranolol on 3,3',5'-triiodothyronine, 3',5'-diiodothyronine, 3,3'-diiodothyronine, and 3'-monoiodothyronine kinetics.

The effect of D,L-propranolol (80 mg daily) on the peripheral monodeiodination of rT3, 3',5'-diiodothyronine (3',5'-T2), 3,3'-diiodothyronine (3,3'-T2), and 3'-monoiodothyronine (3'-T1) was studied in seven out-patients with severe pretreatment hypothyroidism. The patients were maintained euthyroid on a constant L-T4 replacement therapy. A bolus injection technique was used; MCR, production rate (PR), and conversion rate were determined using a noncompartmental kinetic model. During D,L-propranolol, serum rT3 and 3',5'-T2 increased (P less than 0.02), and 3,3'-T2 seemed to decrease. The MCRs of rT3, 3',5'-T2, and 3,3'-T2 (P less than 0.02) decreased during drug treatment. The MCR and PR of 3'-T1 were reduced, albeit not significantly (P less than 0.10). The PR of 3,3'-T2 was reduced (P less than 0.02), whereas the PRs of rT3 and 3',5'-T2 were unaltered. The conversion rate of rT3 to 3',5'-T2 was unaltered. No changes were seen in the apparent distribution volumes of the iodothyronines studied. The results are compatible with the assumption that D,L-propranolol, or a metabolite thereof, inhibits the 5'-deiodination of all of the iodothyronines.