Thyroid Hormone and Lactation in the Rat.

Abstract

Effects of l-thyrox-ine alone, and in combination with oxytocin and growth hormone (GH) upon lactation in the rat were studied. Amount of milk obtained by a litter of 6 during 30 minutes nursing on day 14 postpartum, expressed as percent litter body weight, was used as the index of response. 3.0 [mu]g/100 g/day l-thyroxine, a level equal to the highest daily thyroid secretion rate previously observed, injected daily from days 7-13, increased average milk yield 37%. That obtained with aid of oxytocin, to insure maximum milk removal, in rats injected for 7 days with 1-thyroxine alone, or with 1 mg GH/day, was increased 63.2 and 73.7%, respectively. With addition of each hormone there resulted also a progressive reduction in variation among individual milk yields. It is suggested suboptimal thyroid hormone secretion may limit intensity of milk secretion in many lactating rats. Increased milk secretion, obtained with 1-thyroxine with or without GH was not reflected in significant increases in litter growth as analyzed by percent weight gain during period of treatment. Thyroxine had no adverse effect on maternal body weight. Increase in maternal weight due to GH was offset by simultaneous treatment with thyroxine. Thyroxine induced a significant increase in maternal pituitary gland weight which was offset when administered with GH. Adrenal gland weight was significantly increased in lactating rats injected with 1-thyroxine plug GH but was unaffected by 1-thyroxine alone.