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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