Studies on the conversion of thyroxine to 3,5,3'-triiodothyronine in normal and thyroidectomized chickens

The possibility of conversion of L-thyroxine (T4) to L-3,5,3'-triiodothyronine (T3) and to 3,3',5'-triiodothyronine (reverse-T3, r-T3) has been investigated in chickens. Reverse-T3 could not be detected in normal animals. Both thyroidectomized and normal animals were able to convert exogenous T4 to T3. The administration of propylthiouracyl blocked this conversion to a greater extent than had been expected from the data on mammals. The conversion ratio is higher than in any other species reported in the literature. Data suggest that, as a result of the adaptation of the peripheral deiodination, thyroidectomized animals were able to convert more T4 to T3 than were normal ones.