ADRENAL DISORDERS

Adrenal gland is made up of the cortex and medulla, each functioning as a separate endocrine unit.

Adrenal cortex is functionally divided into three zones-outer zona glomerulosa, middle zona fasciculata, and inner zona reticularis to produce different hor- mones-glucocorticoids (cortisol), mineralocorticoids (aldosterone), and sex steroids.

Glucocorticoids, e.g. cortisol (most potent natural glucocorticoid) are produced in zona fasciculata, under control of pituitary ACTH, which in turn is regulated by hypothalamic CRH.

Mineralocorticoid, i.e. aldosterone, is synthesized in zona glomerulosa. mainly regulated by Renin-angio­tensin system.

Sex-steroids, e.g. androgens and small quantities of estrogens are produced in zona fasciculata and zona reticularis as well as in gonads. Adrenals are main source of sex-steroids in pre-pubertal period, while there is sudden surge of gonadal steroid production at the onset of puberty. Exact physiology of sex-steroid secretions is complex, discussed in Ch 22.6.2.

All adrenocortical hormones are derived from choles­terol via different but inter-related enzymatic pathways (Fig. 22.2). Enzymatic block in one pathway leads to—(a) accumulation of proximal metabolite with or without biological activity, (b) deficiency of end-hormone, i.e. cortisol, aldosterone or androgens, and (c) diversion

Fig. 22.2: Adrenal steroidogenesis.

HSD: Hydroxysteroid dehydrogenase; KSR: Ketosteroid reductase

of intermediate substrate to produce other hormones, leading to their excess.

Adrenal medulla, though structurally a part of adrenal, has totally unrelated biological function, i.e. secretion of direct-acting sympathetic catecholamines, e.g. norepinephrine, epinephrine and dopamine. Its functions are neither related to adrenal cortex, nor regulated by hypothalamo-hypophyseal axis. Pheochromocytoma is the principle disorder of adrenal medulla, discussed later.

22.5.1