Radioactively labeled thyroxine was administered to rats, and after 1 to 20 hr their livers were isolated and perfused with pooled rat blood. It was found that, during the perfusion, unchanged T4 was released to the perfusing blood. The efflux of T4 from liver to blood was decreased by the addition of sodium salicylate to the perfusing blood and was reversed by the addition of 2 mg of stable T4 to the perfusate. When T4 in liver and blood had approached an equilibrium, exchange transfusion resulted in a further efflux of T4 from the liver and the establishment of a new equilibrium. The dynamic equilibrium existing between T4 in liver and in blood was shown most dramatically when T4- 131I was in the liver and T4-125I was in the blood when perfusion of the liver began. Livers from rats given radioactively labeled triiodothyronine (T3) released only a small amount of T3 to the perfusing blood but deiodination and conjugation were greater than after administration of T4-131I. (Endocrinology 79: 391, 1966)
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Gorman et al. (1966) studied this question.