ONTOGENY OF TYPE-I AND TYPE-III DEIODINASE ACTIVITIES IN EMBRYONIC AND POSTHATCH CHICKS - RELATIONSHIP WITH CHANGES IN PLASMA TRIIODOTHYRONINE AND GROWTH-HORMONE LEVELS

1. The ontogeny of type I and type III deiodinase activities was studied in embryonic an posthatch chicks. 2. Hepatic type I activity showed a 3-fold increase up to the period of pipping and hatching and decreased slowly thereafter. 3. Hepatic type III activity increased by 3-fold from E14 to E17 and decreased more than 10-fold from E17 to C0. Posthatch levels were very low. 4. Type I activity in the kidney decreased slowly after hatching while type III activity was very low over the whole period studied. 5. Developmental changes during the late embryonic period suggest a causal relationship between the increase in plasma GH and T3 levels and the decrease in hepatic type III activity.