Influence of Estrogen on the Pituitary-Thyroid System of the Female Rat: Mechanisms and Loci of Action

Abstract

The influence of estrogen on the pituitary-thyroid system of the adult rat was determined after sc administration of small daily doses of estradiol benzoate (EB: .004, .016, .064 and .246 μg; 9–11 days) to 26–28 day ovariectomized animals and after pituitary implantation of 17β-estradiol in rats bearing median eminence lesions (18–39 days). It was demonstrated that estradiol has a biphasic action on TSH secretion and thyroid gland activity. Chronic administration of the .064 μg dose of EB produced thyroid hyperplasia. Lower doses of the steroid had no significant effect on thyroid histology, whereas a 4-fold increase in dose (.256 μg) induced histological regression in the gland (accumulation of colloid with reduction in acinar cell height). TSH blood titers and thyroid activity were markedly diminished in spayed ratswith hypothalamic lesions. Intrahypophysial estradiol implants in such animals stimulated thyroid function and significantly elevated TSH levels. Cholesterol implants were ineffective. The implantation of relatively large amounts of estradiol inhibited pituitary-thyroid function. The results clearly indicate that the ovarian hormone has both stimulatory and inhibitory actions TSH secretion by the adenohypophysis. Physiological levels of estradiol promote TSH secretion; it is suppressed in estrogen deficiency excess. The fact that this biphasic effect on TSH can be elicited in rats with lesions in the median eminence supports the view that estrogen acts at the pituitary rather than at the hypothalamic level to influence thyroid function. (Endocrinology84: 117, 1969)