Abstract
Expt. with 145 rats showed that hypophysectomy in rats is followed by thyroid atrophy which cannot be compensated by sympathetic stimulation (cold). This indicates that the cold reaction of the thyroid is mediated through the ant. pituitary. Unilateral cervical sympathectomy in the rat neither affects the ipsilateral thyroid lobe nor modifies the histological evidence of thyroid activity after thyrotropic stimulation. Bilateral cervical sympathectomy in rats causes a temporary and mild hypoactivity of the thyroid, ap-parently mediated through the ant. lobe. Similarly, sympathectomy modifies the cold reaction temporarily.