PITUITARY ADRENOCORTICOTROPIC ACTIVITY IN ALTERED THYROID FUNCTION

Rats were stimulated by different two triggers of ACTH; unilateral adrenalectomy and SU-4885. The rate of ACTH release in response to those stimuli was evaluated by 3 parameters; plasma corticosterone concentration, pituitary ACTH content, and ascorbic acid concentration. Surgical stress produced higher and more persistently elevated plasma corticosterone concentration in rats administered by triiodothyronine than in the control rats, while the reverse was obvious in thyroidectomized rats. This difference could not be accounted completely by differences in adrenal responsiveness to tropic hormone among them. The findings on the rate of depletion in adrenal ascorbic acid as well as pituitary ACTH clearly indicated that in response to stress the pituitary gland of hyperthyroid rat is more responsive in terms of ACTH release, while that of hypothyroid rat was less responsive than the control rat. Following SU-4885 plasma corticosterone showed to decrease in similar fashion independent of thyroid function. Nevertheless, hyperthyroid rats released pituitary ACTH more rapidly than euthyroid rats, while hypothyroid rats released it more slowly, in response to the depression of plasma corticosterone concentration. It is clear from these results that the thyroid hormone is an important factor which controls the capacity of the pituitary gland to release ACTH in response to stress.