Androgen production requires a complex chain of events. It all begins in the brain, where the hypothalamus produces gonadotropin-releasing hormone (GnRH, also known as luteinizing hormone-releasing hormone, LHRH). Hormones are chemicals that are produced in one part of the body before traveling to another part to do their work. GnRH is a true hormone, but it doesn’t have a long commute; it acts on a nearby part of the brain, the pituitary gland. In response to GnRH, the pituitary secretes two additional hormones, follicle stimulating hormone (FSH) and luteinizing hormone (LH). FSH and LH were named for their effects on ovaries, but they are every bit as important for men; both act on the testicles, where LH triggers testosterone production and FSH, acting with testosterone, stimulates sperm production.
Testosterone is produced by the Leydig cells of the testicles. The starting point is cholesterol, notorious for its effects on the heart but critical as the building block of all sex hormones, male and female.
Testosterone has many direct effects on the male anatomy and metabolism. It is responsible for the deep voice, increased muscle mass, and strong bones that characterize the gender. It stimulates the production of red blood cells by the bone marrow. It also has crucial, if incompletely understood, effects on male behavior; it contributes to aggressiveness and is essential for the libido or sex drive, as well as for normal erection and sexual performance. Testosterone stimulates the growth of the genitals at puberty, and it is responsible for sperm production throughout adulthood. Finally, and for most men unhappily, testosterone also acts on the liver, raising the production of LDL (“bad”) cholesterol.
Although testosterone acts directly on many other tissues, some of its least desirable effects don’t occur until it is converted into another androgen, dihydrotestosterone (DHT). DHT acts on the skin, sometimes producing acne, and on the hair follicles, putting hair on the chest but often taking it from the scalp. Male pattern baldness is one thing, prostate disease quite another—and DHT also stimulates the growth of prostate cells, producing normal growth in adolescence but contributing to benign prostatic hyperplasia (BPH) and prostate cancer in many older men.
About 95% of a man’s testosterone is produced in the testicles under the control of LH. The remaining 5% is produced in the adrenal glands. Women also make testosterone in their adrenal glands in both sexes, adrenal hormone production is independent of LH and FSH. In both men and women, cholesterol is the basis for adrenal androgen production—and in both, an important precursor of testosterone is dehydroepiandrosterone (DHEA), another hormone that is widely popular as a non-prescription dietary supplement.
Testosterone metabolism has a final complexity; in its last throes, the quintessential male hormone is converted to estradiol, a major female hormone. Most of this final conversion takes place in fat cells, which is why obese men (and women) have higher estrogen levels than lean men (and women).