Effect of chronic treatment with estrogen and thyroxine, alone and combined, on the rate of deiodination of L-thyroxine to 3,5,3'-triiodothyronine in vitro.

The effect of administration of thyroxine (T4) and estradiol benzoate (EB), alone and combined (EB + T4), to female rats for 35 days on their serum concentrations of T4 and triiodothyronine (T3), and on the rate of conversion of T4 to T3 by homogenates of their liver and kidneys was studied. Administration of T4 (50 micrograms/kg b.w./day) resulted in an increase, while administration of EB (57 micrograms/kg b.w./day) resulted in a decrease in serum levels of T4. In EB + T4-treated rats, the serum concentration of T4 was less than that of either the T4-treated or control groups, while the serum T3 concentration was greater than that of the EB-treated group. Administration of T4 resulted in a rate of hepatic generation of T3 which was 180% of control, while the rate of renal T3 generation was 152% of control. Activity of T4 5'-monodeiodinase in hepatic homogenates was not affected significantly by administration of EB, while that in renal homogenates was inhibited by 44%. Administration of T4 in combination with EB resulted in a rate of conversion of T4 to T3 by hepatic homogenates which was 334% of control and significantly higher than that achieved by T4 alone. The rate of renal T3 generation by this group was 129% of control, but less than that achieved by administration of T4 alone. Administration of EB + T4 potentiated the effect of T4 on hepatic T4 to T3 converting activity and is likely responsible for the reduction in serum T4 concentration of these animals.