1. Conversion of thyroxine into triiodothyronine and reverse tri-iodothyronine in intact cells was studied with isolated renal tubules prepared by collagenase digestion.
2. Conversion of thyroxine into triiodothyronine and reverse tri-iodothyronine increased progressively for at least 90 min.
3. Studies of tri-iodothyronine production from increasing amounts of thyroxine revealed that the thyroxine to tri-iodothyronine conversion is saturable.
4. Iodide and carbimazole had no effect on the thyroxine to tri-iodothyronine conversion.
5. 6-Propyl-2-thiouracil had a direct noncompetitive inhibitory effect on the conversion of thyroxine into tri-iodothyronine with a 75% inhibition of the conversion at a propylthiouracil concentration within the therapeutic range in vivo. Propylthiouracil also inhibited the net formation of reverse tri-iodothyronine from thyroxine at a similar propylthiouracil concentration, as well as inhibiting the subsequent degradation of reverse triiodothyronine.