The Rate of Thyroxine Secretion by the Thyroid Glands of White Leghorn Cockerels *

The physiology of the thyroid gland has become of increasing interest to us because of the observations made by Graham3 and the confirmation by others that the feeding of dried thyroid gland or the injection of thyroxine would dramatically increase the milk yield and fat secretion of many dairy cows. The effect of thyroxine upon the rate of growth and body type and conformation has been suggested by many studies (see Koger and Turner6). It is interesting that Andrews and Bullard' found the fattening of steers to be enhanced, at least for a time, by partial thyroidectomy. These observations suggest that a slight hyperthyroid condition compared to the normal would be favorable for maximum milk and butterfat productiion in cattle and for more rapid growth in a number of species of animals. On the other hand "a slightly hypothyroid condition would be conducive to fattening. As intense lactation and rapidity of growth are characteristics of good dairy stock and fattening is characteristic of beef stock, it would appear that the normal rate of thyroxine secretion might play an important role in determining the intensity of milk secretion, on the one hand, and of fattening qualities of cattle, on the other hand. To determine the degree of hyperthyroidism necessary for optimum milk and butterfat production and the degree of hypothyroidism necessary for most efficient fattening, it is necessary to determine the normal rate of thyroxine secretion in individuals and groups of animals varying in these characteristics. Methods of estimating the thyroxine secretion of individuals or groups of animals have not been developed except by replacement therapy in thyroidectomized animals. The use of thyroid-inhibiting compounds related to thiourea permits a new approach to this problem. Before attempting to apply the method to the larger domestic animals, it seemed desirajble to test it with various breeds of chickens,