Thyroxine dissociation in human plasma: measurement of its rate by a continuous-flow dialysis method

A method is described for measuring the rate at which thyroxine dissociates from binding proteins in human plasma. The method depends upon the fact that free thyroxine is able to diffuse through Visking dialysis membrane whereas bound thyroxine cannot. At 20 degrees C half of the plasma thyroxine dissociates within about 200 seconds and 75% within 10 min. At 37 degrees C half of the hormone dissociates in less than 20 seconds and the reaction is virtually complete in 3-4 min. These values agree well with those previously obtained by a resin-uptake method. In the Discussion it is calculated that the pool of free thyroxine in the plasma is replinished about 5 times 10-6 times each day and that an individual thyroxine molecule spends only about 15 mseconds in free solution during each transit.