Steroids

Naturally Occurring Steroids and Their Functions in Humans

The defining structural feature of all steroids is a skeleton of carbon atoms arranged into four fused rings called the steroid nucleus. The cellular ability to form the steroid nucleus is found principally in more complex nucleus-containing cells, rather than in bacterial cells, which lack a nucleus. Steroids are found in fungi, plants, and animals.

In humans, steroids perform various important functions. Among them are the control of membrane fluidity by cholesterol, the facilitation of calcium absorption by vitamin D, the regulation of mineral excretion by aldosterone, and the establishment of stress responses by cortisol. Sex hormones are steroids. Androgens are male sex hormones. Estrogens are female sex hormones. The major androgen is testosterone; the major estrogen is estradiol.

The primary effect of sex hormones is the development of the gonads to enable the formation of the reproductive cells—spermatozoa in males and oocytes in females. The secondary effects in females are breast development, broadening of the pelvis, increased fat beneath the skin, and growth of pubic and armpit hair. In males, the muscle mass increases, the skeleton gets bigger and stronger, the larynx enlarges and the voice deepens, the skin coarsens, and hair growth occurs over much of the body.

Some steroids can be made naturally by humans, chiefly in the ovaries or testes with smaller amounts formed in the adrenal glands. Transformations of sex hormones are sometimes important. One of the most surprising is that male brain development, which is initiated by the presence of testosterone, occurs after it is converted to estradiol. Aldosterone and cortisol are synthesized in the adrenal glands. Sunlight drives the formation of vitamin D in the skin. Then the liver and kidneys transform it into the biologically useful form. Too little vitamin D may be synthesized during the short days of winter, so additional vitamin must be acquired from food or supplements.

Pharmacologie Uses of Naturally Occurring and Synthetic Steroids

Ovulation in human females is inhibited by high levels of estradiol and progesterone, which block the release of hormones from the hypothalamus and the pituitary to promote ovulation. Natural or synthetic hormones may be used to prevent ovulation, thus making pregnancy impossible.