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

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Abstract

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.

Introduction

The overall function of the adrenals is to protect the organism against acute and chronic stress. In acute stress, catecholamines mobilise glucose and fatty acids for energy and prepare the heart, lungs and muscles for action. Glucocorticoids have their main function during chronic stress, e.g. control of the intermediary metabolism with a predominance on catabolism of proteins and fat in order to obtain an increased gluconeogenesis. Gluconeogenesis in homeotherms is especially critical during prolonged fasting since circulating glucose and stores of liver glycogen will be used up in less than 24 h, at least in mammals [30].

In contrast to glucocorticoids, thyroid hormones may be viewed as tissue growth factors which increase both synthesis and degradation of proteins, with a predominance on synthesis which is responsible for normal body growth. Indeed, normal overall whole body growth does not occur in the absence of thyroid hormones despite adequate levels of growth hormone [27].

Section snippets

Inhibition of the thyroidal axis by glucocorticoids

In general, hormones of the adrenal axis inhibit thyroid function in all adult vertebrates studied. The following is a more detailed account on this phenomenon in fish and birds.

Activation of the thyroidal axis by glucocorticoids

Several activating effects of interrenal or adrenal glands on thyroid function have been described. In the Japanese flounder (Paralichthys olivaceus) cortisol enhances the effect of thyroid hormones on fin ray shortening in vitro, but is ineffective in the absence of thyroid hormones [13]. In Salvelinus fontinalis [44]and in Cyprinus carpio [1]cortisol stimulates the in vitro conversion of T4 into T3, while in O. mykiss no effect was found [2]. In foetal sheep, there is a coincident rise in the

Thyroidal axis

The implication of the thyroid gland in amphibian metamorphosis has been described almost a century ago by Gudernatch [28]. Subsequent studies have lead to the Etkin [18]hypothesis of metamorphosis. Briefly, premetamorphic anuran tadpoles acquire a hypothalamic sensitivity to thyroid hormones (TH) (T4 or T3) resulting in prolactin inhibition and activation of the thyroidal axis. A rapid decrease in circulating prolactin and an increase in thyrotropin releasing hormone (TRH) will consequently

Thyroidal axis

Thyroid hormones are essential for successful development and for the hatching process in the chicken. They are involved in yolk retraction and prepare the animal for the drastic switch from an allantoic to a pulmonary respiration by inducing the functional maturation of the lungs [12].

Towards the end of incubation, precocial birds have higher plasma T4 and T3 levels, coinciding with increased GH concentrations from day 19 on. The increase of plasma T3 levels is negatively correlated to a

Conclusion

In conclusion, inhibitory effects of the adrenal axis on thyroid function are confined mainly to adult vertebrates. During embryonic or larval development CRH may stimulate the release of TSH, next to ACTH, and consequently the release of T4, whereas glucocorticoids are mainly involved in peripheral control mechanisms in order to obtain high plasma levels of T3.

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