The administration of thyroxine increased the activity of hepatic microsomal Δ⁴-reductase for testosterone and hydrocortisone in male rats. In contrast, the thyroxine treatment slightly decreased the hepatic microsomal steroid Δ⁴-reductase activity in female rats. The Δ⁴-reductase activity of liver supernatant fraction of male and female rats was not affected by the thyroxine treatment. Castration markedly increased the microsomal steroid Δ⁴-reductase activity in male rats and treatment with testosterone depressed the steroid Δ⁴-reductase activity to the level of intact male rats. The administration of thyroxine into these castrated and castrated plus testosterone-treated rats did not increase the activity of microsomal steroid Δ⁴-reductase.
On the other hand, there was no sex difference in the activity of hepatic microsomal steroid Δ⁴-reductase in mice and the thyroxine treatment slightly decreased the steroid Δ⁴-reductase activity both in male and fenale mice.
These results suggested that the increase in the steroid Δ⁴-reductase activity in hepatic microsomes of male rats by the thyroxine treatment may be related to an impairment of androgen-induced decrease in the steroid Δ⁴-reductase activity in rat liver microsomes.