THE EFFECT OF AMIODARONE ON THYROXINE KINETICS

Abstract

Amiodarone was given by daily intraperitoneal injection (10 mg/kg body weight) to a group of 11 New Zealand white rabbits. Over 6 weeks there were significant increases (P less than 0.01) in plasma total T4 concentration (43 +/- 8 to 60 +/- 13 nmol/l; mean +/- 1 SD) and rT3 concentration (0.12 +/- 0.06 to 0.31 +/- 0.16 nmol/l) and a significant fall in total plasma T3 (2.3 +/- 0.3 to 1.7 +/- 0.2 nmol/l). The plasma clearance of T4 in rabbits treated with amiodarone for 6 weeks was significantly reduced relative to controls (64 +/- 25 vs 109 +/- 19 ml/kg per day, P less than 0.01) but the production rate was unchanged (3.8 +/- 1.8 vs 3.7 +/- 1.4 nmol/kg per day). In the amiodarone treated animals the increased plasma T4 concentration was entirely accounted for by the reduced clearance. It is inferred that the normal output of T4 by the thyroid is pituitary dependent and that TSH secretion is sustained in the presence of hyperthyroxinaemia because of amiodarone induced partial inhibition of T4 utilization by the thyrotroph.