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Effect of propranolol on extrathyroidal metabolism of thyroxine and 3,3',5-triiodothyronine evaluated by noncompartmental kinetics.

Kinetic studies of T4 and T3 using a noncompartmental approach were performed in seven patients with pretreatment severe hypothyroidism maintained on L-T4 replacement. Each subject received a combined tracer dose of labeled T4 and T3 as an iv bolus before and during peroral treatment with propranolol. Serum T4 was unchanged, while a significant decrease of 13% was found in serum T3. The disposal rates (DR) of T4 and T3 decreased significantly, and the ratio between the DR off T3 and the DR of T4, the conversion rate, was significantly reduced during propranolol treatment. The decrease in the DR of T4 suggests a reduction in the bioavailability of L-T4 during propranolol, possibly due to a decrease in intestinal absorption. The decrease in the conversion rate indicates a reduced extrathyroidal conversion of T4 to T3 during propranolol treatment.