Modulation of estrogen-induced peroxidase activity in the rat uterus by thyroid hormones

The effect of thyroid hormones, iodide, propylthiouracil and other thyroid-active agents on estrogen receptor concentration and on the induction of peroxidase in the immature rat uterus by estradiol (E 2) was examined. T 3 (0.5 mg/kg) given daily for 6 days produced a large decrease in E 2-induced uterine peroxidase but lowered only slightly the concentration of cytosolic or nuclear receptors in this organ. T 4 also decreased the effect of E 2 on uterine peroxidase induction and the inhibitory action of T 3 was observed in thyroidectomized rats. The administration of iodide (10 mM) in the drinking water for 6 days caused a 2-fold increase in estrogen-induced peroxidase in both normal and thyroidectomized immature animals without influencing the estrogen receptor concentration in the uterus. The effect of other thyroid-active agents on serum T 3 and T 4 levels was also determined. Treatment for 6 days with propylthiouracil (6 mM) increased the concentration of estrogen receptors in the uterus and decreased serum T 3 and T 4 levels without any effect on uterine peroxidase activity while bromide and perchlorate did not influence these parameters. Possible mechanisms for the inhibitory effects of thyroid hormones on uterine peroxidase induction by E 2 are discussed.