Steroids are lipid-soluble, hydrophobic molecules synthesized from cholesterol. They can be classified into six categories, based on their primary biological activity. An example of each category is shown in Figure 31.2.
Glucocorticoids, such as cortisol, are primarily produced in cells of the adrenal cortex and regulate processes involved in glucose, protein, and lipid homeostasis. Glucocorticoids generally produce effects that are catabolic in nature. Aldosterone, a primary example of a mineralocorti-coid, is produced in cells of the outermost portion of the adrenal cortex. Aldosterone is primarily involved in regulating sodium and potassium balance by the kidneys and is the principal mineralocorticoid in the body.
Androgens, such as testosterone, are primarily produced in the testes, but physiologically significant amounts can be synthesized by the adrenal cortex as well. The primary female sex hormone is estradiol, a member of the estrogen family, produced by the ovaries and placenta. Progestins, such as progesterone, are involved in maintenance of pregnancy and are produced by the ovaries and placenta.
The calciferols, such as 1,25-dihydroxycholecalciferol, are involved in the regulation of calcium homeostasis. 1,25-dihydroxycholecalciferol is the hormonally active form of vitamin D and is formed by a sequence of reactions occurring in skin, liver, and kidneys.
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