The permanence of tissue binding of thyroxine in the rat.

1. Male rats were given radiothyroxine and after 24 hr, approximately half the blood volume was replaced by exchange transfusion.2. Plasma concentration of radiothyroxine 2 hr after exchange averaged 81.1% of the value before exchange. This compared with a predicted mean of 97.1% assuming a free and rapid exchange of thyroxine throughout the thyroxine distribution space. The difference between the observed and expected value was highly significant.3. The effective exchangeable pool of thyroxine was found to be approximately twice the plasma volume.4. From these and other observations it is suggested that the liver and plasma constitute most of the exchangeable pool of thyroxine.5. This experiment supports a model of thyroxine metabolism which envisages essentially permanent binding of thyroxine by tissues other than the liver.