The influence of thiouracil and thiourea on the growth and fattening of lambs.

T HE role of the thyroid gland in the regulation of general body metabo, [ism, metamorphosis, growth, and development is well known.The in. fluence of the thyroid gland and its hormone, thyroxine, upon the amount and distribution of body fat is not thoroughly understood. Although obesity is frequently associated with hypothyroidism in the human, this is not always the case and some hypothyroid humans have very little subcutaneous fat. However, thyroidectomy, and the reduction of thyroid activity with thiouracil (Astwood, 1943; and others) is often followed by an increase in body weight in the human. Andrews and Bullard (I94o) and Bullard and Andrews (I943) found that beef steers gained at an increased rate following partial thyroidectomy. The weight gains were markedly increased during the first six to eight postoperative weeks but were subsequently reduced to approximately preoperative levels following hypertrophy and hyperplasia of the thyroid gland remnants. Richter and CIisby (i942), Kennedy (i94~), Astwood and co-workers (~943) (I94~), MacKenzie and MacKenzie (~943), and McGinty and By. water (i945) have demonstrated that a large number of substances related to thiourea inhibit thyroxine secretion, decrease basal metabolic rate and result in thyroid hypertrophy and hyperplasia in the rat. Two of these effective compounds, thiourea and thiouracil, are available in quantities sufficient for large animal experimentation and were used in this investigation. Turner et al. (1944) reported that thiourea and thiouracil will completely stop thyroxine secretion in cattle, sheep and goats when sufficient quantities of the drugs are fed. Kempster and Turner (i94~) found that when New Hampshire chicks werefed a ration containing o.2 percent thiouracil for a ~6-day period beginning at io weeks of age, growth rate was unaffected but market grade was ;reproved, and tl:,at when thiouracil was fed at this level for 36 days feed requirements per pound of gain were increased, growth rate was depressed, but market quality was greatly improved. Andrews and Schnetzler (I946) found that when thiouracil was fed at a level of 0.20 percent to Barred Plymouth Rock chickens between the 6th and I4th weeks growth was slightly retarded but market grade and fat deposition