Testosterone
Testosterone
Testosterone is a steroid hormone from the androgen group. Testosterone is primarily secreted in the testes of males and the ovaries of females although small amounts are secreted by the adrenal glands. It is the principal male sex hormone and an 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. On average, the adult male body produces about twenty to thirty times the amount of testosterone that an adult female’s body does
Production
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 thecal cells of the ovaries, by the placenta, as well as by the zona reticularis of the adrenal cortex in both sexes.
In the testes, testosterone is produced by the Leydig cells. 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).
A 2001 study[2] examined the effect of a 3-week period of sexual abstinence followed by masturbation-induced orgasm. It found that abstinence over such periods “does not change the neuroendocrine response to orgasm but does produce elevated levels of testosterone in males.”
A 2003 study[3] showed that serum testosterone levels reach a peak seven days after abstaining from ejaculation
Virilizing and anabolic effects 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 linear 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 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 androgen effects
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
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
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 odour
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
Advanced postnatal 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.
Phallic enlargement (including clitoromegaly)
Increased libido and frequency of erection or clitoral engorgement
Pubic hair extends to thighs and up toward umbilicus
Facial hair (sideburns, beard, moustache)
Chest hair, periareolar hair, perianal hair
Subcutaneous fat in face decreases
Increased muscle strength and mass
Deepening of voice
Growth of the adam’s apple
Growth of spermatogenic tissue in testes, male fertility
Growth of jaw, brow, chin, nose, and remodeling of facial bone contours
Shoulders widen 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.
