Increased heat loss in ovariectomized hypothyroid rats treated with estradiol

The role of the thyroid in the mediation of an estrogen-associated change in thermal balance was studied in thyroidectomized and in propylthiouracil-treated ovariectomized rats. Prior to propylthiouracil treatment, estrogen-treated ovariectomized rats and intact female rats had higher rates of heat production and dry heat loss at -5 degrees C than ovariectomized rats. Heat production of estrogen-treated and intact female rats was well below their rates of dry heat loss without an alteration in the absolute rate of heat loss in the hypothyroid condition. Heat production exceeded heat loss only in the hypothyroid ovariectomized group not receiving estrogen. Ovariectomized rats without estrogen maintained thermal balance, whereas rectal temperatures fell in both intact and estrogen-treated ovariectomized rats during cold exposure. Increased heat loss unbalanced by heat production was also observed in surgically thyroidectomized estrogen-treated ovariectomized rats tested at -5 degrees C. These results suggest that an estrogen-induced increase in heat loss, which is compensated by an increase in heat production in the euthyroid but not the hypothyroid condition, is one mechanism responsible for estrogen-associated changes in thermal balance during cold exposure.