The unidentified glucuronide previously described in dog bile or in urine of the dehepatized dog following the administration of I131-thyroxine (T4) has been identified chromatographically- as 3,3′,5′ triiodothyronine glucuronide. 3,3′,5′ Triiodothyronine (3,3′,5′ T3) also was found in smaller amounts in the free state in bile and in the plasma of the dehepatized dog. Some of this compound was conjugated with sulfate, and some was found in an unidentified acid-labile conjugate. Formation of 3,3′,5′ T3 from thyroxine (T4) involves removal of an iodine atom from the phenolic ring, and thus is the same type of reaction noted earlier in the conversion of 3,5,3′ triiodothyronine (T3) to 3,3′ diiodothyronine. Loss of this iodine atom results in biologic inactivation of these thyroid hormones. Another metabolite of T4 was found to be 3,3′ T2 which was conjugated with sulfate. Formation of this substance involves the removal of one iodine atom from each of the rings. In spite of this evidence for removal of I131 from the ring with the alanine side chain by extrahepatic tissues, deiodination of this ring of T4 is much less complete in dehepatized dogs than in dogs with livers as shown by the decreased output of radioactive iodide in the urine of the dehepatized dog.