Abstract
The main secretory product of the thyroid gland is thyroxine (T4), 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 (T3) occurs in the presence of outer-ring (ORD) and inner-ring deiodinases (IRD), respectively. During ontogenesis high plasma levels of T3 occur at parturition or hatching in precocial mammals and birds and at metamorphic climax in amphibians. These high levels may be obtained by increased T4 ORD activity and also by decreased T3 IRD activity resulting in an inhibition of T3 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 T4 ORD but inhibits T3 IRD activity and T3 degradation.
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