Effect of Thyroxine Administration on the 11β-Hydroxysteroid Dehydrogenases in Rat Liver and Kidney

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Abstract
The activity of the TPN/DPNdependent enzyme system which interconverts cortisol and cortisone was diminished in the livers of rats which had received injections of L-thyroxine. Although the level of activity was lower in females than in males, and decreased with age in both sexes, thyroxine administration invariably decreased the activity regardless of age and sex. The lowered activity was also observed when adrenalectomized rats were subjected to thyroxine treatment. Thyroxine had no effect on the activity when incubated with liver homogenate in vitro. The concentration of enzyme in livers from thyroidectomized animals was higher than in the control group. A variety of 11β-hydroxysteroids and 11-ketosteroids were substrates for the enzyme system and the activity toward these was decreased by thyroxine treatment. A similar system in rat kidney which interconverts cortisol and cortisone was unchanged by thyroxine administration. The effect of thyroxine on this enzyme system in rat liver is the reverse of that thought to be operative in man. A possible rationalization of this is discussed. (Endocrinology75: 592, 1964)