Evidence for a direct effect of thyroid hormones on the hepatic synthesis of estrogen receptors in the rat.

The role of thyroid hormones in the regulation of estrogen receptor turnover in the rat liver was studied. Animals subjected to thyroidectomy or hypophysectomy in combination with different hormone substitutions, were used. The receptor level in control animals was 53 fmol/mg cytosol protein. Thyroidectomy for 28 days caused a dramatic reduction to 20 fmol/mg, whereas hypophysectomy for 9 days resulted in an even more substantial reduction to 11 fmol/mg protein. If animals, hypophysectomized for 9 days, were given triiodothyronine (T3) for 9 days the hepatic estrogen receptor concentration was elevated to 22.5 fmol/mg protein. Estradiol given together with T3 did not cause any further increase in the receptor level. We conclude that thyroid hormones affect the hepatic synthesis of estrogen receptors on two levels, via a direct action on the liver and via an indirect modulation of the pituitary hormone synthesis/release.