Ontogeny of type I and type III deiodinase activities in embryonic and posthatch chicks: relationship with changes in plasma triiodothyronine and growth hormone levels.
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1. The ontogeny of type I and type III deiodinase activities was studied in embryonic and 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.