Serum levels and kinetics of digoxin in patients with hyperthyroidism (author's transl)

The mechanism of reduced sensitivity to digitalis in patients with hyperthyroidism has been attributed to a change of intrinsic myocardial function and/or to altered pharmacokinetics of cardiac glycosides. Digoxin kinetics have been studied in hyperthyroid and 8 euthyroid patients after a single oral and i.v. dose at steady-state. Plasma and urinary digoxin concentrations were determined by radioimmunoassay. A significantly mean lower serum digoxin concentration was found in hyperthyroid patients, both after a single oral drug administration and at the steady-stage. A decreased digoxin absorption could not account for this finding, since both the percentage of gastrointestinal uptake of the drug and maximal serum concentration did not differ in hyperthyroid patients as compared to controls. Hyperthyroid subjects showed, on the contrary, an expended distribution volume and a significantly higher excretion, as documented by a lower drug half-life, by the increase of the elimination constant and urinary digoxin output.