Regulation of the Conversion of Thyroxine to Triiodothyronine in the Perfused Rat Liver

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Abstract

This study was undertaken to determine what factors control the conversion of thyroxine (T4) to triiodothyronine (T3) in rat liver under conditions approximating those found in vivo. Conversion of T4 to T3 was studied in the isolated perfused rat liver, a preparation in which the cellular and structural integrity is maintained and that can perform most of the physiologic functions of the liver. The perfused liver readily extracted T4 from perfusion medium and converted it to T3. Production of T3 by the perfused liver was a function of the size of the liver, the uptake of T4 by the liver, and the presence of T4-5′-deiodinase activity. Production of T3 was increased by increasing the uptake of T4 by liver, which could be accomplished by increasing the liver size, by increasing the perfusate T4 concentration, or by decreasing the perfusate albumin concentration. These changes occurred without altering the conversion of T4 to T3. The liver had a large capacity for extracting T4 and for T4-5′-deiodination to T3, which was not saturated at a T4 concentration of 60 μg/dl. Production of T3 was decreased by inhibiting hepatic T4-5′-deiodinase with propylthiouracil, which decreased T3 production by decreasing the conversion of T4 to T3. Propylthiouracil did not alter hepatic T4 uptake.

Authors

Anthony S. Jennings, Duncan C. Ferguson, Robert D. Utiger