Testosterone

Testosterone Physiological Effects

In general, androgens promote protein synthesis and growth of those tissues with androgen receptors. Testosterone effects can be classified as virilizing and anabolic, although the distinction is somewhat artificial, as many of the effects can be considered both. Testosterone is anabolic, meaning it builds up bone and muscle mass.

  • ''Anabolic effects'' include growth of muscle mass and strength, increased bone density and strength, and stimulation of linear growth and bone maturation.
  • ''Androgenic effects'' include maturation of the sex organs, particularly the penis and the formation of the scrotum in the fetus, and after birth (usually at puberty) a deepening of the voice, growth of the beard and axillary hair. Many of these fall into the category of male secondary sex characteristics.

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.

Prenatal

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); although the role of testosterone is far smaller than that of Dihydrotestosterone.
  • Development of prostate and seminal vesicles
  • Gender identity

Early infancy

''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. Surprisingly, the male brain is masculinized by testosterone being aromatized into estrogen, which crosses the blood-brain barrier and enters the male brain, whereas female fetuses have alpha-fetoprotein which binds up the estrogen so that female brains are not affected.

Pre-peripubertal

''Pre- Peripubertal effects'' are the first visible effects of rising androgen levels at the end of childhood, occurring in both boys and girls.

  • Adult-type body odour
  • Increased oiliness of skin and hair, acne
  • Pubarche (appearance of pubic hair)
  • Axillary hair
  • Growth spurt, accelerated bone maturation
  • Hair on upper lip and sideburns.

Pubertal

''Pubertal effects'' begin to occur when androgen has been higher than normal adult female levels for months or years. In males, these are usual late pubertal effects, and occur in women after prolonged periods of heightened levels of free testosterone in the blood.

  • Enlargement of sebaceous glands. This might cause acne.
  • Phallic enlargement or clitoromegaly
  • Increased libido and frequency of erection or clitoral engorgement
  • Pubic hair extends to thighs and up toward umbilicus
  • Facial hair (sideburns, beard, moustache)
  • Loss of scalp hair (Androgenetic alopecia)
  • Chest hair, periareolar hair, perianal hair
  • Leg hair
  • Axillary hair
  • Subcutaneous fat in face decreases
  • Increased muscle strength and mass
  • Deepening of voice
  • Increase in height
  • Growth of the Adam's apple
  • Growth of spermatogenic tissue in testicles, male fertility
  • Growth of jaw, brow, chin, nose, and remodeling of facial bone contours
  • Shoulders become broader and rib cage expands
  • Completion of bone maturation and termination of growth. This occurs indirectly via estradiol metabolites and hence more gradually in men than women.

Adult

''Adult testosterone effects'' are more clearly demonstrable in males than in females, but are likely important to both sexes. Some of these effects may decline as testosterone levels decrease in the later decades of adult life.

  • Libido and clitoral engorgement/penile erection frequency
  • Regulates acute HPA response under dominance challenge
  • Mental and physical energy
  • Maintenance of muscle trophism
  • The most recent and reliable studies have shown that testosterone does not cause or produce deleterious effects on prostate cancer. In people who have undergone testosterone deprivation therapy, testosterone increases beyond the castrate level have been shown to increase the rate of spread of an existing prostate cancer.
  • Recent studies have shown conflicting results concerning the importance of testosterone in maintaining cardiovascular health. Nevertheless, maintaining normal testosterone levels in elderly men has been shown to improve many parameters which are thought to reduce cardiovascular disease, risk such as increased lean body mass, decreased visceral fat mass, decreased total cholesterol, and glycemic control.
  • Under dominance challenge, may play a role in the regulation of the fight-or-flight response

Testosterone is necessary for normal sperm development. It activates genes in Sertoli cells, which promote differentiation of spermatogonia.

  • Studies show that falling in love decreases men's testosterone levels while increasing women's testosterone levels. It is speculated that these changes in testosterone result in the temporary reduction of differences in behavior between the sexes.
  • Recent studies suggest that testosterone level plays a major role in risk taking during financial decisions.
  • Fatherhood also decreases testosterone levels in men, suggesting that the resulting emotional and behavioral changes promote paternal care.

In animals (grouse and sand lizards), higher testosterone levels have been linked to a reduced immune system activity. Testosterone seems to have become part of the honest signaling system between potential mates in the course of evolution.

Brain

As testosterone affects the entire body (often by enlarging; men have bigger hearts, lungs, liver, etc.), the brain is also affected by this "sexual" advancement.

A study conducted in 1996 found no immediate short term effects on mood or behavior from the administration of supraphysiologic doses of testosterone for 10 weeks on 43 healthy men.

Literature suggests that attention, memory, and spatial ability are key cognitive functions affected by testosterone in humans. Preliminary evidence suggests that low testosterone levels may be a risk factor for cognitive decline and possibly for dementia of the Alzheimer’s type, a key argument in Life Extension Medicine for the use of testosterone in anti-aging therapies. Much of the literature, however, suggests a curvilinear or even quadratic relationship between spatial performance and circulating testosterone, where both hypo- and hypersecretion of circulating androgens have negative effects on cognition and cognitively-modulated aggressivity, as detailed above.

Contrary to what has been postulated in outdated studies and by certain sections of the media, aggressive behaviour is not typically seen in hypogonadal men who have their testosterone replaced adequately to the eugonadal/normal range. In fact, aggressive behaviour has been associated with hypogonadism and low testosterone levels and it would seem as though supraphysiological and low levels of testosterone and hypogonadism cause mood disorders and aggressive behaviour, with eugondal/normal testosterone levels being important for mental well-being. Testosterone depletion is a normal consequence of aging in men. One consequence of this is an increased risk for the development of Alzheimer’s Disease.

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Testosterone Biochemistry

Biosynthesis

Like other steroid hormones, testosterone is derived from cholesterol (see figure to the right). The first step in the biosynthesis involves the cleavage of the sidechain of cholesterol by CYP11A, a mitochondrial cytochrome P450 oxidase with the loss of six carbon atoms to give pregnenolone. In the next step, two additional carbon atoms are removed by the CYP17A enzyme in the endoplasmic reticulum to yield a variety of C19 steroids. In addition, the 3-hydroxyl group is oxidized by 3-β-HSD to produce androstenedione. In the final and rate limiting step, the C-17 keto group androstenedione is reduced by 17-β hydroxysteroid dehydrogenase to yield testosterone.

The largest amounts of testosterone (>95%) are produced by the testes in men. The male generative glands also contain Sertoli cells which require testosterone for 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).

Regulation

In males, testosterone is primarily synthesized in Leydig cells. The number of Leydig cells in turn is regulated by luteinizing hormone (LH) and follicle stimulating hormone (FSH). In addition, the amount of testosterone produced by existing Leydig cells is under the control of LH which regulates the expression of 17-β hydroxysteroid dehydrogenase.

Environmental factors affecting testosterone levels include:

  • Loss of status or dominance in men.
  • Implicit power motivation predicts an increased testosterone release in men.
  • Aging reduces testosterone release.
  • Hypogonadism
  • Sleep (REM dream) increases nocturnal testosterone levels.
  • Resistance training increases testosterone levels, however, in older men, that increase can be avoided by protein ingestion.
  • Zinc deficiency lowers testosterone levels but over supplementation has no effect on serum testosterone.
  • Licorice. The active ingredient in licorice root, glycyrrhizinic acid has been linked to small, clinically non-significant decreases in testosterone levels. In contrast, a more recent study found that licorice administration produced a substantial testosterone decrease in a small, female-only sample.

Metabolism

Approximately 7% of testosterone is reduced to 5α-dihydrotestosterone (DHT) by the cytochrome P450 enzyme 5α-reductase, an enzyme highly expressed in male accessory sex organs and hair follicles. an enzyme expressed in the brain, liver, and adipose tissues.

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Testosterone - What is Testosterone?

Testosterone is a steroid hormone from the androgen group. In mammals, testosterone is primarily secreted in the testes of males and the ovaries of females, although small amounts are also secreted by the adrenal glands. It is the principal male sex hormone and an anabolic steroid. Testosterone is evolutionarily conserved through most vertebrates, although fish make a slightly different form called 11-ketotestosterone.

In men, testosterone plays a key role in the development of male reproductive tissues such as the testis and prostate as well as promoting secondary sexual characteristics such as increased muscle and bone mass and hair growth. In addition, testosterone is essential for health and well-being as well as preventing osteoporosis.

On average, an adult human male body produces about ten times more testosterone than an adult human female body, but females are, from a behavioral perspective (rather than from an anatomical or biological perspective), more sensitive to the hormone. However, the overall ranges for male and female are very wide, such that the ranges actually overlap at the low end and high end respectively.

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Testosterone Mechanism

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 5α-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 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. It is important to note that if there is a 5-alpha reductase deficiency, the body (of a human) will ''continue'' growing into a female with testicles.

Androgen 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 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.

The human hormone testosterone is produced in greater amounts by males, and less by females. The human hormone estrogen is produced in greater amounts by females, and less by males. Testosterone causes the appearance of masculine traits (i.e., deepening voice, pubic and facial hairs, muscular build, etc.) Like men, women rely on testosterone to maintain libido, bone density and muscle mass throughout their lives. In men, inappropriately high levels of estrogens lower testosterone, decrease muscle mass, stunt growth in teenagers, introduce gynecomastia, increase feminine characteristics, and decrease susceptibility to prostate cancer, reduces libido and causes erectile dysfunction and can cause excessive sweating and hot flushes. However, an appropriate amount of estrogens is required in the male in order to ensure well-being, bone density, libido, erectile function, etc.

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