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Testosterone is the principle male sex hormone and is responsible for reproductive growth and development in male vertebrates.
Testosterone either activates androgen receptors in its unchanged form or gets converted to 5α-dihydrotestosterone (DHT) by the enzyme 5α-reductase and then binds to androgen receptors. Once bound, the receptor-hormone complex moves into the cell nucleus and binds to specific genes sequences on the cellular DNA called hormone response elements. This modifies the DNA transcription and synthesis of various proteins, thereby giving rise to the androgenergic effects exerted by testosterone.
Testosterone may also be converted to the female sex hormone estradiol, the most important estrogen in female reproductive development and bone health.
Testosterone plays a major role in the growth and development of the male reproductive organs such as the testes and prostate.
Some specific effects of testosterone include:
Anabolic effects of testosterone - Testosterone is the primary anabolic steroid. It promotes an increase is muscle mass and strength. Testosterone is also responsible for the mass, density and strength of bone. In males, an age-related decline in testosterone increases the risk of bone disorders such as osteoporosis.
Androgenergic effects - The growth of primary sexual characteristics such as the penis and testes are mediated by testosterone both while the baby forms in the womb and during puberty.
Secondary sexual characteristics mediated by testosterone include deepening of the voice and growth of facial, armpit, chest and pubic hair.
The effects of testosterone vary according to age. Some of the age-related effects of testosterone include:
During the 7th to 12th week of pregnancy, the fetus forms the gonads and the scrotum and penis develop. The primary effects on the growth of these reproductive organs are exerted by DHT rather than the unchanged form of testosterone.
The levels of testosterone remain high during the first few months of life, before declining to pre-pubertal blood levels by 4 to 6 months.
As a child approaches puberty, testosterone produces several developmental characteristics in both boys and girls such as body odour, acne and a spurt in bone growth that lengthens the limbs and gives height.
During puberty, an increase in testosterone levels in boys causes enlargement of the reproductive organs such as the penis and testes, increased libido, increased frequency of erection, and the growth of facial, chest, nipple and pubic hair. Some maturing boys may experience a loss of hair on the scalp, referred to as alopecia.
The fat content beneath the skin also decreases, while muscle mass increases, as do height and rib cage size. The voice also deepens, and the jaws, chin and nose start to take on a more adult appearance. The Adam's apple in the throat also becomes more prominent.
Testosterone maintains male fertility, libido, mental and physical energy levels, muscle strength, general well being and prevents depression and fatigue.