The exchange of thyroxine between plasma and liver: the role of free thyroxine

An investigation has been made into the exchange of thyroxine between human plasma and the perfused rat liver. A stop-flow method was used to hold plasma within the liver sinusoids for periods up to 20 min. At 20 degrees C it was found that thyroxine present in the sinusoids transferred to the liver at a rate of 5-6%/min; and that thyroxine transferred from the liver to the plasma at a rate of 1%/min. In a second series of experiments, trace amounts of free thyroxine were injected into the stream of plasma just as it entered the tissue; the liver captured 18.5% of this injected hormone. In the discussion it is noted that 29% of the plasma thyroxine dissociates into free solution each minute. If the liver is able to capture 18.5% of this hormone (as the above experiment suggests) then the plasma-to-liver transfer rate would be 5-6%/min, which is the rate actually observed. These observations strongly suggest that the transfer of thyroxine from plasma to liver involves its initial release into the free state followed by trapping by the tissue binding sites.