Influence of Thyroxine on the Metabolism of Amino Acids and Proteins during Fasting *

Following thyroidectomy the catabolism of nitrogen is accelerated, as indicated by studies both on fed and fasting rats.' "8 In an earlier report evidence was presented which showed that after thyroidectomy the rate of amino acid catabolism is elevated, while at the same time, a fall in the rate of conversion of body protein to amino acids occurs. The nitrogen output of fasting hypothyroid animals was found to be slightly greater than normal, indicating that of these two effects of thyroidectomy on the metabolism of nitrogen, the influence on amino acid catabolism is predominant. The experiments to be reported in this communication deal with the effects of small doses of thyroxine on the amino acid and protein metabolism of fasting thyro-parathyroidectomized rats. By labelling the amino acid reservoir of these animals with isotopic nitrogen (N15), it has been possible to show that the daily injection of 10 ug. of dl-thyroxine for 7 days is sufficient to repress the catabolism of the tissue amino acids of fasting hypothyroid animals, while at the same time, an acceleration of protein breakdown occurs.