Metabolic Effects of Thyroxine Injected Into Normal, Thiouracil-Treated and Thyroidectomized Rats1

Since the rat has classically been considered an animal remarkably refractory to thyroxine, this report is being written to point out the great change in sensitivity produced by thyroidectomy. Credit must be given Meyer and Wertz (1939) for making similar observations 10 years ago, using orally administered thyroxine, but their results have long been ignored, possibly because of the claim of 25 to 30 fold increases in responsiveness. Young adult male albino rats of the Sprague-Dawley strain, weighing 280–350 grams, were used in this study. They were placed on the limited feeding regime used in this laboratoryin which food intake is restricted to 5 hours per day, resulting in an essentially post-absorptive animal which was available each day for metabolic study. Such rats were carefully trainedin the Benedict (1930) multiple-chamber, closed-circuit apparatus for determination of oxygen consumption. When thiouracil was administered, 2 gm. of the drug were thoroughly mixed into each kg. of powdered Rockland Rat Diet.