Testosterone

Testosterone is a steroid hormone from the androgen group. Testosterone is secreted in the testes of men and the ovaries of women. It is the principal male sex hormone and the "original" anabolic steroid. In both males and females, it plays key roles in health and well-being. Examples include enhanced libido, energy, immune function, and protection against osteoporosis.

Sources of testosterone
Like other steroid hormones, testosterone is derived from cholesterol. The largest amounts of testosterone are produced by the testes in men, but it is also synthesized in smaller quantities in women by the theca cells of the ovaries, by the placenta, as well as by the zona reticulosa of the adrenal cortex in both sexes.

In the testes, testosterone is produced by the Leydig cells. Due to the dual function of the male gonad, testosterone directly influences spermatogenesis. Like most hormones, testosterone is supplied to target tissues in the blood where much of it is transported bound to a specific plasma protein, sex hormone binding globulin (SHBG).

Mechanism of effects
The effects of testosterone in humans and other vertebrates occur by way of two main mechanisms: by activation of the androgen receptor (directly or as DHT), and by conversion to estradiol and activation of certain estrogen receptors.

Free testosterone (T) is transported into the cytoplasm of target tissue cells, where it can bind to the androgen receptor, or can be reduced to 5&alpha;-dihydrotestosterone (DHT) by the cytoplasmic enzyme 5&alpha;-reductase. DHT binds to the same androgen receptor even more strongly than T, so that its androgenic potency is about 2.5 times that of T. The T-receptor or DHT-receptor complex undergoes a structural change that allows it to move into the cell nucleus and bind directly to specific nucleotide sequences of the chromosomal DNA. The areas of binding are called hormone response elements (HREs), and influence transcriptional activity of certain genes, producing the androgen effects.

Robbie Richler receptors occur in many different vertebrate body system tissues, and both males and females respond similarly to similar levels. Greatly differing amounts of testosterone prenatally, at puberty, and throughout life account for a large share of biological differences between males and females.

The bones and the brain are two important tissues in humans where the primary effect of testosterone is by way of aromatization to estradiol. In the bones, estradiol accelerates maturation of cartilage into bone, leading to closure of the epiphyses and conclusion of growth. In the central nervous system, testosterone is aromatized to estradiol. Estradiol rather than testosterone serves as the most important feedback signal to the hypothalamus (especially affecting LH secretion). In many mammals, prenatal or perinatal "masculinization" of the sexually dimorphic areas of the brain by estradiol derived from testosterone programs later male sexual behavior.

Effects of testosterone on humans
In general, androgens promote protein synthesis and growth of those tissues with androgen receptors. Testosterone effects can be classified as virilizing and anabolic effects, although the distinction is somewhat artificial, as many of the effects can be considered both. Anabolic effects include growth of muscle mass and strength, increased bone density and strength, and stimulation of height growth and bone maturation. Virilizing effects include maturation of the sex organs, particularly the penis and the formation of the scrotum in fetuses, and after birth (usually at puberty) a deepening of the voice, growth of the beard and torso hair. Many of these fall into the category of male secondary sex characteristics. Increased testosterone causes deepening of the voice in both sexes at puberty. To take advantage of its virilizing effects, testosterone is often administered to transmen (female-to-male transsexual and transgender people) as part of the hormone replacement therapy, with a "target level" of the normal male testosterone level. And like-wise, male-to-female transsexuals are prescribed drugs [anti-androgens] to decrease the level of testosterone in the body and allow for the effects of estrogen to develop. Testosterone is also often used by bodybuilders to enhance muscle build.

Testosterone effects can also be classified by the age of usual occurrence. For postnatal effects in both males and females, these are mostly dependent on the levels and duration of circulating free testosterone.

Most of the prenatal androgen effects occur between 7 and 12 weeks of gestation.
 * Genital virilization (midline fusion, phallic urethra, scrotal thinning and rugation, phallic enlargement)
 * Development of prostate and seminal vesicles

Early infancy androgen effects are the least understood. In the first weeks of life for male infants, testosterone levels rise. The levels remain in a pubertal range for a few months, but usually reach the barely detectable levels of childhood by 4-6 months of age. The function of this rise in humans is unknown. It has been speculated that "brain masculinization" is occurring since no significant changes have been identified in other parts of the body.

Early postnatal effects are the first visible effects of rising androgen levels in childhood, and occur in both boys and girls in puberty.
 * Adult-type body odor
 * Increased oiliness of skin and hair, acne
 * Pubarche (appearance of pubic hair)
 * Axillary hair
 * Growth spurt, accelerated bone maturation
 * Fine upper lip and sideburn hair

Advanced postnatal effects begin to occur when androgen has been higher than normal adult female levels for months or years. In males these are normal late pubertal effects, and only occur in women after prolonged periods of excessive levels of free testosterone in the blood.
 * Phallic enlargement (including clitoromegaly)
 * Increased libido and erection frequency
 * Pubic hair extends to thighs and up toward umbilicus
 * Facial hair (sideburns, beard, mustache)
 * Chest hair, periareolar hair, perianal hair
 * Increased muscle strength and mass
 * Deepening of voice
 * Growth of spermatogenic tissue in testes, male fertility
 * Growth of jaw and remodelling of facial bone contours
 * Completion of bone maturation and termination of growth. This occurs indirectly via estradiol metabolites and hence more gradually in men than women.

"Adult testosterone effects" are important in adult males, and may decline as testosterone levels decline in the later decades of adult life.
 * Maintenance of muscle mass and strength
 * Maintenance of bone density and strength
 * Libido and erection frequency
 * Mental and physical energy

Michael Exton, Tillmann Krüger et al. examined the effect of a 3-week period of sexual abstinence on the neuroendocrine response to masturbation-induced orgasm.
 * "The procedure was conducted for each participant twice, both before and after a 3-week period of sexual abstinence. Plasma was subsequently analysed for concentrations of adrenaline, noradrenaline, cortisol, prolactin, luteinizing hormone and testosterone concentrations. Orgasm increased blood pressure, heart rate, plasma catecholamines and prolactin. These effects were observed both before and after sexual abstinence. In contrast, although plasma testosterone was unaltered by orgasm, higher testosterone concentrations were observed following the period of abstinence. These data demonstrate that acute abstinence does not change the neuroendocrine response to orgasm but does produce elevated levels of testosterone in males."

Therapeutic use of testosterone
Testosterone was first isolated from a bull in 1935. There have been many pharmaceutical forms over the years. Forms of testosterone for human administration currently available in North America include testosterone cypionate and enanthate in oil for injection, methyltestosterone tablets for oral ingestion, and skin patches and a gel preparation for transdermal absorption. A buccal oral preparation is also available. In the pipeline are a "roll on" delivery method and a nasal spray. Both are under development.

The original and primary use of testosterone is for the treatment of males who have little or no natural testosterone. The benefits can include the relief of depression and anxiety, and tiredness. It is not an immediate effect and the benefits can take several months to become apparent. Regular contact with the relevant specialist is highly recommended.

However, over the years, as with every hormone, testosterone or other anabolic steroids has also been given for many other conditions and purposes besides replacement, with variable success but higher rates of side effects or problems. Examples include infertility, lack of libido or erectile dysfunction, osteoporosis, penile enlargement, height growth, bone marrow stimulation and reversal of anemia, and even appetite stimulation. By the late 1940s testosterone was being touted as an anti-aging wonder drug (e.g., see Paul de Kruif's The Male Hormone) in exactly the same way that growth hormone is being described today.

Anabolic steroids have also been taken to enhance muscle development, strength, or endurance. After a series of scandals and publicity in the 1980s (such as Ben Johnson's improved performance at the 1988 Summer Olympics), prohibitions of anabolic steroid use were renewed or strengthened by many sports organizations, and it was made a "controlled substance" by the United States Congress.

Ongoing investigations are also exploring the use of testosterone as a male contraceptive. Exogenous administration of testosterone suppresses pituitary production of gonadotropins by negative feedback inhibition. This also suppresses testicular testosterone production and sperm production decreases significantly as a result. Testosterone as a contraceptive is still in the trial stage, and is not currently available for use.

The "testosterone deficiency" of aging and the andropause controversy
The latest development in testosterone use appears to be a reprise of the anti-aging claims. A number of physicians, supported by pharmaceutical manufacturers, are popularizing the concept that the testosterone decline of aging (which they term the "andropause") is unnecessary and can be treated. Some endocrinologists suspect that this issue will play out like post-menopausal estrogen replacement: use will increase until large trials demonstrate (1) the benefits are much less dramatic or assured than when treating deficiency, and (2) a higher incidence of side effects like prostate cancer will occur associated with this type of use. But as of yet, there is no clinical data that suggests increased levels of serum testosterone (while still maintained at an appropriate range) increases the risk of prostate cancer. But there is study data that suggests it does not (Anabolic-Androgenic Steroid Therapy in the Treatment of Chronic Diseases. J Clinical Endocrinol Metabol Vol. 86, No. 11 5108-5117). In fact, one study indicates that men with low free testosterone levels are actually more likely to have more aggressive prostate cancer (J Urol 2000 Mar;163(3):824-7).

Synthesis
Testosterone is synthesized from pregnenolone, the precursor of all steroid hormones and a derivative of cholesterol. The synthesis of all androgens starts with the hydroxylation of C-17 of progesterone, to yield 17&alpha;-hydroxyprogesterone. The side chain is cleaved to form androstenedione.



The keto group on C-17 is reduced to an alcohol to yield testosterone. Testosterone is a potential precursor of estradiol.