steroid hormone

steroid hormone, any of a group of hormones that belong to the class of chemical compounds known as steroids; they are secreted by three “steroid glands”—the adrenal cortex, testes, and ovaries—and during pregnancy by the placenta. All steroid hormones are derived from cholesterol. They are transported through the bloodstream to the cells of various target organs where they carry out the regulation of a wide range of physiological functions.

These hormones often are classified according to the organs that synthesize them: the adrenal steroids are so called because they are secreted by the adrenal cortex, and the sex hormones are those produced by the ovaries and testes. This distinction is not exclusive, however, because the adrenal cortex also secretes sex hormones, albeit to a lesser extent than do the gonads, and the ovaries under abnormal conditions may produce adrenal steroids.

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The adrenal cortex produces the adrenocortical hormones, which consist of the glucocorticoids and the mineralocorticoids. Glucocorticoids such as cortisol control or influence many metabolic processes, including the formation of glucose from amino acids and fatty acids and the deposition of glycogen in the liver. Glucocorticoids also help to maintain normal blood pressure, and their anti-inflammatory and immunosuppressive actions have rendered them useful in treating rheumatoid arthritis and preventing the rejection of transplanted organs. Mineralocorticoids such as aldosterone help maintain the balance between water and salts in the body, predominantly exerting their effects within the kidney.

The androgens are the male sex hormones. The principal androgen, testosterone, is produced primarily by the testes and in lesser amounts by the adrenal cortex and (in women) by the ovaries. Androgens are primarily responsible for the development and maintenance of reproductive function and stimulation of the secondary sex characteristics in the male. Androgens also have an anabolic (synthesizing and constructive, rather than degradative) function in stimulating the production of skeletal muscles and bone as well as red blood cells. To enhance the anabolic activity of androgens without increasing their masculinizing ability, anabolic steroids were developed. Though originally intended to combat diseases marked by wasting, these synthetic hormones have been abused by individuals desiring to increase their muscle mass, such as athletes seeking to gain a competitive advantage. Overdosing has been linked to serious side effects, including infertility and coronary heart disease.

Estrogens are one of the two types of female sex hormones. They are secreted mainly by the ovaries and in smaller amounts by the adrenal glands and (in men) by the testes. Estradiol is the most potent of the estrogens. Functioning similarly to androgens, the estrogens promote the development of the primary and secondary female sex characteristics; they also stimulate linear growth and skeletal maturation. In other mammals these hormones have been shown to precipitate estrus (heat). The ovarian production of estrogen plummets during menopause.

Progestins, the most important of which is progesterone, are the other type of female sex hormone and are named for their role in maintaining pregnancy (pro-gestation). Estrogens and progestins are secreted cyclically during menstruation. During the menstrual cycle, the ruptured ovarian follicle (the corpus luteum) of the ovary produces progesterone, which renders the uterine lining receptive to the implantation of a fertilized ovum. Should this occur, the placenta becomes the main source of progesterone, without which the pregnancy would terminate. As pregnancy progresses, placental production of progesterone increases, and these high doses suppress ovulation, preventing a second conception. The contraceptive quality of progesterone led to the development of structurally modified progestins and estrogens—the oral contraceptives known as birth-control pills, used by women to prevent unwanted pregnancy.