Inhibition and activation of the thyroidal axis by the adrenal axis in vertebrates

Hormones of the adrenal or interrenal axis and stress situations which induce elevated glucocorticoid plasma levels (e.g. handling and starvation), inhibit thyroid function in growing and adult vertebrates. However, data indicate that during foetal and embryonic development (mammals and birds) or during larval growth and metamorphosis (fish and amphibians), the adrenal axis may stimulate thyroid function. Recent findings have provided some information concerning this stimulatory interference of the adrenal axis. In amphibians corticotropin releasing hormone and not thyrotropin releasing hormone is thyrotropic during metamorphosis, thus providing the substrate T4 necessary for T3 production. Other data indicate that the increase in plasma T3 at metamorphic climax may be the result of an inhibition of the T3 degrading activity, rather than stimulation of the T4 into T3 converting activity, and that glucocorticoids may be responsible for this. Also, in the chick embryo glucocorticoids effectively increase plasma T3 concentration by reducing the hepatic T3 degrading activity, whereas corticotropin releasing hormone also induces an elevation in the thyrotropin plasma levels and hence raises T4 concentrations which may function as a substrate for T3 production.