Effect of Dihydrotachysterol on TSH Secretion in Rats

Crystalline dihydrotachysterol (DHT) stimulates the activity of the thyroid gland in puberal female rats. After i.p. injection of 350 or 500 µg/kg/day DHT, the height of epithelial thyroid cells increased, resulting in an increase of thyroid cells from 19 to 26% of the total follicle area. The secretory activity of the gland increased, serum TSH rising from 1.1 to 2.1 mU/ml and serum thyroxine rising from 2.5 to 6 µg%. The basal metabolic rate increased from 1,400 to 1,810 ml O2/kg/h oxygen consumption. Body weight, however, did not differ from that of the control group. At a dosage of 750 µg/kg/day, the above reaction was accompanied by a body weight drop of 20%. Within 4 h after implantation of crystalline DHT into the anterior basal hypothalamus, the thyroid reacted histologically with the above pattern of an active gland. These changes could not be elicited in hypophysectomized rats. These results would indicate that the stimulatory effect of DHT on the thyroid gland acts via the hypothalamo-pituitary axis.