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Male Reproductive Endocrinology
Male sexual development and hormonal function depend on a complex feedback circuit involving the hypothalamus-pituitary-testes modulated by the central nervous system. Male sexual dysfunction (see Male Sexual Dysfunction) can be secondary to hypogonadism, neurovascular disorders, drugs, or other disorders.
The hypothalamus produces gonadotropin-releasing hormone (GnRH), which is released in a pulsatile fashion every 60 to 120 min. Its target organ, the anterior pituitary gland, responds to each pulse of GnRH by producing a corresponding pulse of luteinizing hormone (LH) and, to a lesser degree, follicle-stimulating hormone (FSH). If the GnRH pulses do not occur with the proper amplitude, frequency, and diurnal variation, hypogonadism may result. (idiopathic hypogonadotropic hypogonadism). Continuous (as opposed to pulsatile) stimulation by GnRH agonists (eg, as a treatment for advanced prostate cancer) actually suppresses pituitary release of LH and FSH and thus testosterone production.
The Leydig cells of the testes respond to LH by producing between 5 and 10 mg of testosterone daily. Testosterone levels are highest in early morning and lowest during the evening hours; however, in older men, this diurnal pattern may be blunted.
Testosterone is synthesized from cholesterol through several intermediate compounds, including dehydroepiandrosterone (DHEA) and androstenedione. Circulating testosterone is mostly protein-bound, about 40% avidly bound to sex hormone–binding globulin (SHBG) and 58% loosely bound to albumin. Thus, only about 2% of circulating testosterone is bioavailable as free testosterone.
In target tissues, about 4 to 8% of testosterone is converted to a more potent metabolite, dihydrotestosterone (DHT), by the enzyme 5α-reductase. DHT has important trophic effects in the prostate and mediates androgenic alopecia. In adults, spermatogenesis requires adequate intratesticular testosterone, but the role of DHT in spermatogenesis is unclear.
Testosterone and DHT have metabolic and other effects, including
Stimulating protein anabolism (increasing muscle mass and bone density)
Stimulating renal erythropoietin production (increasing red blood cell mass)
Stimulating bone marrow stem cells (modulating the immune system)
Causing cutaneous effects (ie, sebum production, hair growth)
Causing neural effects (ie, affecting cognition, increasing libido and possibly aggression)
Testosterone undergoes conversion to estradiol as well as to DHT; estradiol mediates most of testosterone's action on organs such as bones and the brain.
Testosterone, DHT, and estradiol provide negative feedback on the hypothalamic-pituitary axis. In males, estradiol is the main inhibitor of LH production, whereas both estradiol and inhibin B, a peptide produced by Sertoli cells of the testes, inhibit production of FSH. In the presence of testosterone, FSH stimulates the Sertoli cells and induces spermatogenesis. In spermatogenesis, each germinal cell (spermatogonium), located adjacent to the Sertoli cells, undergoes differentiation into 16 primary spermatocytes, each of which generates 4 spermatids. Each spermatid matures into a spermatozoon. Spermatogenesis takes 72 to 74 days and yields about 100 million new spermatozoa each day. Upon maturation, spermatozoa are released into the rete testis, where they migrate to the epididymis and eventually to the vas deferens. Migration requires an additional 14 days. During ejaculation, spermatozoa are mixed with secretions from the seminal vesicles, prostate, and bulbourethral glands.
In the embryo, the presence of a Y chromosome triggers development and growth of the testes, which begin secreting testosterone and a müllerian duct inhibitor by about 7 wk of gestation. Testosterone virilizes the wolffian duct (which develops into the epididymis, vas deferens, and seminal vesicles). DHT promotes development of the external genitals. Testosterone levels peak in the 2nd trimester and fall to almost zero by birth. Testosteroneproduction rises briefly during the first 6 mo of life, after which testosterone levels remain low until puberty.
LH and FSH are elevated at birth but fall to low levels within a few months, remaining low or undetectable throughout the prepubertal years. Through an unknown mechanism, blood levels of the adrenal androgens DHEA and DHEA sulfate begin to increase several years before puberty. Their conversion to testosterone in small amounts initiates pubic and axillary hair growth (adrenarche). Adrenarche can occur as early as 9 or 10 yr of age.
The mechanisms that initiate puberty are unclear, although early in puberty the hypothalamus becomes less sensitive to the inhibitory effects of sex hormones. This desensitization increases secretion of LH and FSH, corresponding to pulsatile GnRH secretion, and stimulating testosterone and sperm production. In boys, the increased testosterone levels cause pubertal changes, the first of which is growth of the testes and scrotum. Later, penile length, muscle mass, and bone density increase; the voice deepens; and pubic and axillary hair becomes denser and thicker (see Figure: Puberty—when male sexual characteristics develop.).
Both hypothalamic secretion of GnRH and the response of Leydig cells to FSH and LH diminish with aging. In the elderly, Leydig cells decrease in number as well. Beginning at about age 30, a man’s serum total testosterone levels decline by 1 to 2%/yr. Men aged 70 to 80 tend to have serum testosterone levels that are about one half to two thirds of those of men in their 20s. In addition, SHBG levels increase with aging, causing an even greater decline in serum free and bioavailable testosterone. FSH and LH levels tend to be normal or high-normal. These age-related changes are referred to as the andropause, although there are no abrupt changes in hormone levels (and corresponding symptoms) as occur in the menopause. The decline in testosterone may contribute to a combination of symptoms that has been termed androgen deficiency of the aging male (ADAM), which includes
If men have these symptoms plus low serum testosterone, they are diagnosed with hypogonadism (see Male Hypogonadism) and are eligible for treatment with supplemental testosterone.
Testosterone supplementation in men with low-normal levels of testosterone is controversial. Some experts recommend a trial of testosterone supplementation in older men with symptoms or signs of hypogonadism and whose serum testosterone levels are slightly below the lower limit of normal. No data favor any of the testosterone preparations specifically for use in ADAM, although daily transcutaneous applications appear to be the most physiologic and best tolerated.
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