Abstract
ABSTRACT In all vertebrates the main secretory product of the thyroid gland is thyroxine (T4). The rate of its conversion to the active triiodothyronine (T3) or to the inactive reverse T3 (rT3) can be regulated in the peripheral tissues by deiodination. Probably all vertebrates are capable of both outer and inner ring deiodination (ORD and IRD), catalyzed by deiodinases. Most of these enzymes are related to one of the three mammalian deiodinase types, but the abundance and tissue localization can differ substantially among vertebrate groups. During ontogeny high plasma levels of T3 occur around the period of birth in mammals, of hatching in birds, and of metamorphosis in amphibians. This T3 increase seems to be not only the result of increased T3 production (ORD), but also in considerable degree of decreased T3 degradation (IRD). In fish, birds, and mammals growth hormone can increase plasma T3 levels. In chickens this effect is obtained not by increasing the amount of active hepatic T3-producing (type I)...
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