THYROID REGULATORY FUNCTION OF INTRA-OCULAR PITUITARY TRANSPLANTS1

Abstract

Thyroid regulatory responses to thyroxine administration and to hemithyroidectomy have been studied in hypophysectomized rats bearing intraocular pituitary transplants. The intra-ocular graft, though separated from direct contact with the hypothalamus stimulated thyroid I¹³¹ uptake but did not influence other measures of thyroid function (I¹³¹ release rate, thyroid weight, PBJ). The grafts released ACTH but had no effect on growth or on testicular weight. In the presence of the graft but not in ungrafted hypophysectomized animals hemi-thyroidectomy was followed by increased TSH release as evidenced by a more rapid I131 release rate, stimulation of I¹³¹ uptake and thyroid enlargement. Thyroxine injection led to thyroid inhibition both in pituitary graft-bearing hypophysectomized and in normal animals, but not in ungrafted hypophysectomized rats. These findings were taken to indicate that pituitary tissue is capable of appropriate and autonomous TSH secretory responses to alterations in blood thyroid hormone level. It was also shown that thyroxine treatment depresses thyroid function to a greater degree than does hypophysectomy. The low I¹³¹ release rate of hypophysectomized rats is not further reduced by thyroxine. In contrast, pituitary graft bearing hypophysectomized rats (whose resting thyroid release rate is the same as those animals without the pituitary graft) have a marked thyroid inhibitory response to thyroxine. Maximum decrease in thyroid function after thyroxine administration thus appears to require the presence of pituitary tissue.