The mechanism of thyroxine transfer from plasma to tissue binding sites.

1. An examination has been made of the uptake of thyroxine by rat livers perfused with very dilute solutions of thyroxine-binding globulin (TBG) and thyroxine-binding pre-albumin (TBPA).2. Under these circumstances the rate of transfer of thyroxine to the liver is very similar to the rate of thyroxine dissociation for both TBG and TBPA.3. It is concluded that thyroxine transfer from plasma to tissue-binding sites involves its initial release into the free state and not, as has been suggested, directed exchange.