The steroid hormones are synthesized in the adrenal cortex, the gonads, and the placenta; are all derived from cholesterol and many are of clinical importance. Steroid hormones are synthesized in the mitochondria and smooth endoplasmic reticulum. Because they are lipophilic, they cannot be stored in vesicles from which they would diffuse easily and are therefore synthesized when needed as precursors. Upon stimulation of the parent cell, steroid hormone precursors are converted to active hormones and diffuse out of the parent cell by simple diffusion as their intracellular concentration rises.Because all steroid hormones are derived from cholesterol, they are not soluble in plasma and other body fluids. As a result, steroids are bound to transport proteins that increase their half-life and insure ubiquitous distribution. The protein-bound steroids are in equilibrium with a small fraction of free steroids, which are 'active.' Steroids can act quickly, by binding to cell surface receptors, or slowly, by binding to cytoplasmic or nucleic receptors and ultimately activate gene transcription.The adrenal glands are composed of the adrenal medulla and the adrenal cortex. The adrenal cortex is divided into three major anatomic zones: the zona glomerulosa, which produces aldosterone; and the zonae fasciculata and reticularis, which together produce cortisol and adrenal androgens. The medulla synthesizes catecholamines. More than 30 steroids are produced in the adrenal cortex; they can be divided into three functional categories: mineralocorticoids, glucocorticoids, and androgens.The steroids that are made almost exclusively in the adrenal glands are cortisol, 11-deoxycortisol, aldosterone, corticosterone, and 11-deoxycorti-costerone. Most other steroid hormones, including the estrogens, are made by the adrenal glands and the gonads [
MineralocorticoidsThe mineralocorticoids are formed in the zona glomerulosa. The main function of the mineralocorticoids is to promote tubular reabsorption of sodium and secretion of potassium and hydrogen ions at the collecting tubule, distal tubule, and collecting ducts [2]. When sodium is reabsorbed, water is absorbed simultaneously. The absorption of sodium and water increases fluid volume and arterial pressure.Aldosterone is the most potent mineralocorticoid and accounts for about 90% of the total mineralocorticoid activity. Mineralocorticoid potency in descending order is: aldosterone, 11-deoxycorticosterone, 18-oxocortisol, corticosterone, and cortisol [1]. Although cortisol has mainly glucocorticoid activity, it also has some mineralocorticoid activity. Cortisol has 1/400 of the potency of aldosterone, but its concentration is about 80 times greater than that of aldosterone [4]. The adrenal production of cortisol is approximately 25 mg/day and that of aldosterone is 100μg/day. Corticosterone has mainly glucocorticoid activity and some mineralocorticoid activity.Aldosterone secretion is regulated primarily by the renin-angiotensin system; it also is stimulated by increased serum potas...