Netherlands Journal of Zoology vol:45 issue:1-2 pages:135-139
The main secretory product oi the thyroid gland is thyroxine (T-4) which is a relatively inactive prohormone serving as a substrate for deiodination processes in peripheral tissues. Activation and inactivation of 3,5,3'-triiodo-L-thyronine (T-3) occurs in the presence of outer-ring (ORD) and inner-ring deiodinases (IRD), respectively. During ontogenesis high plasma levels of T-3 occur at parturition or hatching in precocial mammals and birds and at metamorphic climax in amphibians. These high levels may be obtained by increased T-4 ORD activity and also by decreased T-3 IRD activity resulting in an inhibition of T-3 degradation. In mammals, birds and fish growth hormone (GH) has been described as a stimulatory agent for ORD activity. In the chick embryo, GH does not increase the amount of T-4 ORD but inhibits T-3 IRD activity and T-3 degradation.