Testosterone
Basic Information 
Physiology
• Testosterone or 17-beta-hydroxy-4-androstene-3-one is a steroid hormone from the androgen group. It is primarily secreted by the testes in the male and ovaries in the female, but small amounts are also secreted by the adrenal glands.
• In general, testosterone and other androgens promote protein synthesis and growth of those tissues with androgen receptors. More specifically, the effects of testosterone cause an increase in muscle mass and strength, an increase in bone density and strength and stimulation of linear growth. Additionally, it is the principal male sex hormone responsible for the development of secondary male sexual characteristics.
• Testosterone, similar to other steroid hormones, is derived from cholesterol. The testicle produces the largest amount of testosterone by Leydig cells. In females, thecal cells in the ovaries are also capable of producing testosterone, but in far smaller quantities than in males. Additionally, testosterone can be produced by the placenta and by the zona reticularis of the adrenal cortex in both sexes.
• The clinical effects of testosterone are the result of activation of the androgen receptor and by conversion of testosterone to estradiol with subsequent activation of certain estrogen receptors Most testosterone circulates in the blood bound to sex hormone binding globulin (SHBG). 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 5α reductase. DHT binds to the same androgen receptor with a greater affinity than testosterone (its androgenic potency is approximately 2.5 times that of testosterone). The steroid receptor complex then translocates to the nucleus and binds directly to specific nucleotide sequences of the chromosomal DNA, altering transcriptional activity of certain genes.
• Estradiol exerts its influence primarily in bones and brain tissue. Testosterone is aromatized under the action of aromatase to estradiol. Estradiol serves as the most important feedback signal to the hypothalamus.
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