title: Effect of thyroid-suppressive doses of triiodothyronine on thyroxine turnover and on the free thyroxine fraction.
abstract: The relationship between free thyroxine concentration and thyroxine turnover was studied during thyroid suppression with triiodothyronine. Although there was some increase in the proportion of serum thyroxine bound to thyroxine-binding globulin, the ratio of ultrafilterable to protein-bound hormone was not significantly affected. The fractional disappearance rate of thyroxine increased from an average control value of 11.47%/day to 14.72%/day. Because of contraction of the thyroxine distribution space the clearance of thyroxine was less markedly affected, increasing from 1.37 to 1.56 liters/day. Since the ratio of thyroxine turnover to free thyroxine concentration, i.e., the free thyroxine clearance, increased proportionately (4.79-5.55 liters x 10(3)/day) we conclude that triiodothyronine stimulates thyroxine clearance by a mechanism that is independent of effects on free thyroxine concentration.
mesh_terms: D001797:Blood Protein Electrophoresis; D003956:Dialysis; D006801:Humans; D007456:Iodine Isotopes; D012709:Serum Albumin; D012712:Serum Globulins; D013961:Thyroid Gland / Q000187:drug effects*; D013974:Thyroxine / Q000097:blood / Q000378:metabolism*; D013975:Thyroxine-Binding Proteins; D014284:Triiodothyronine / Q000494:pharmacology*
chemical_list: D007456:Iodine Isotopes; D012709:Serum Albumin; D012712:Serum Globulins; D013975:Thyroxine-Binding Proteins; D014284:Triiodothyronine; D013974:Thyroxine