THE ROLE OF THE GASTROINTESTINAL TRACT, INCLUDING THE LIVER, IN THE METABOLISM OF RADIOTHYROXINE*

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Abstract
Immature male rats were injd. with physiological quantities of radioactive l-thyroxine. The injd. material was promptly distributed in the blood (38%), liver (30%), and the remaining tissues (32%). The material then rapidly entered the gastro-intestinal tract, chiefly by the bile but also by direct secretion. At equilibrium the stomach, small and large bowel contained 50% of the circulating radiothyroxine or its derivatives. A massive recirculation of I131 occurred from the bowel. More than 100%/hr. of the radioactive material was secreted into the bowel and was excreted as feces at 3%/hr. 2/3 of the injd. I131 was ultimately excreted in feces and 1/3 in the urine. Thyroxine or its derivatives was removed from the blood by fixation in the liver, kidneys and other tissues at the rate of 1%/hr. More than 1/2 the residual I131 was in the liver 16 days following injn.