Interaction of Furosemide with Serum Thyroxine Binding Sites: In Vivo and in Vitro Studies and Comparison with Other Inhibitors

Abstract

The diuretic furosemide inhibits serum protein binding of T4 in equilibrium dialysis, dextran-charcoal, and competitive ligand binding separation systems and displaces [125I]T4 from isolated preparations of T4-binding globulin (TBG), prealbumin, and albumin. Equilibrium dialysis studies of undiluted normal serum showed that about 10 (μg/ml furosemide increased the free T4 and free T3 fractions. Displacement occurred at lower drug concentrations in sera with infnormal albumin and TBG levels. Binding of [14C]furosemide to TBG was inhibited by unlabeled T4, suggesting that furosemide and T4 share a common binding site. A single oral dose of 500 mg furosemide given to five patients maintained on..