COMPARATIVE ASPECTS OF MATURATION AND CONTROL OF THYROID HORMONE DEIODINATION DURING DEVELOPMENT

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) enzyme, but by inactivation or degradation of hepatic T₃-degrading (type III) enzyme.