Amiodarone alters thyroxine transfer and distribution in humans.

Previous studies indicate that increased serum total and free T4 levels may be secondary to a proportionally greater decrease in serum T4 clearance rates than in production rates after short-term amiodarone administration, to increased T4 production rates as well as reduced serum clearance rates in selective hyperthyroxinemia without overt hyperthyroidism following chronic amiodarone administration, and to a relatively greater increase in T4 production rates than in clearance rates in classical hyperthyroidism. To further evaluate amiodarone-induced alterations of T4 metabolism, serum T4 transfer and distribution were evaluated by compartmental analysis of T4 kinetic studies from eight normal subjects receiving short-term amiodarone or an equivalent amount of iodide, five patients with selective hyperthyroxinemia induced by chronic amiodarone therapy (n = 4) or ioxithalamic acid (n = 1), and five with classical hyperthyroidism. The model consisted of rapidly and slowly equilibrating pools exchanging with serum, with all losses occurring from the tissue pools. Short-term amiodarone administration reduced the fractional T4 transfer rates between serum and the rapidly equilibrating pool to 82% of baseline. In selective hyperthyroxinemia the fractional rates of T4 transfer between serum and both extravascular pools were increased sixfold, whereas minimal alterations were present in the hyperthyroid group. The serum equivalent volume of T4 distribution in the slow pool was significantly reduced following short-term amiodarone, whereas serum and rapid pool volumes were reduced in selective hyperthyroxinemia and slow pool volume was increased in hyperthyroidism.(ABSTRACT TRUNCATED AT 250 WORDS)