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Testosterone is a steroidal hormone secreted by specialised cells in the testes.

Testosterone is classified as an androgen, which means it is a hormone that has masculinizing effects. Other androgenic hormones are produced by the testes and are also produced by other organs in the body, such as the adrenal glands. These glands are located above the kidneys and produce androgenic hormones in both men and women. Testosterone is produced by the testes in the male foetus throughout gestation, where it promotes the formation of the penis, scrotum, prostate gland and other parts of the male reproductive system. It also causes the testes to descend into the scrotum (the testes form within the body and travel down to the scrotum within the two months before birth). Testosterone production continues until about 10 weeks after birth. At this stage, Testosterone production virtually ceases until the male child reaches puberty, when Testosterone production causes the growth of facial and pubic hair, changes in body shape, deepening of the voice and increased muscle mass and strength.

Testosterone production is controlled by a structure within the brain known as the pituitary gland, which receives signals from other parts of the brain when Testosterone production must be increased.

Testosterone production declines with age in men and in some cases, medical treatment to replace this hormone may be required. Signs and symptoms of Testosterone deficiency include: decreased sex drive, erectile dysfunction, lowered sperm count, increased breast size (gynaecomastia), hot flushes, irritability, difficulty concentrating and depression.

Certain genetic diseases such as Klinefelter's syndrome, Kallman's syndrome and Prader-Willi syndrome can even cause abnormalities in Testosterone production in young men.

Testosterone is one of the 'muscle bulking' agents that are abused by bodybuilders, but it is also a very important substance medicinally for the treatment of androgen deficiency diseases.

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