The rate of triiodothyronine dissociation from binding sites in human plasma

A method is described for estimating the rate of triiodothyronine (T3) dissociation from binding sites in human plasma. At trace concentration about 25% of the hormone dissociates rapidly, the reaction being more than half complete within 1 second. The remaining 75% dissociates exponentially with a t1/2 of 4.2 seconds at 37-38 degrees C. When the plasma is loaded with thyroxine to displace the T3 from thyroxine-binding globulin, the slow exponential reaction is lost and all of the hormone dissociates rapidly. These results indicate that the free and bound T3 in plasma are in a state of rapid exchange.