Testosterone | Research & Encyclopedia Articles

Testosterone

Testosterone is a male sex hormone (androgen) that is principally produced by the testes. Testosterone is an important component the normal development of male sexual organs and prostate. Elevated levels of testosterone during puberty cause the development of secondary sexual characteristics (broadening of shoulders, beard development, etc.). Testosterone plays an important role in the growth and development of bone and muscle, and in the maintenance of muscle strength.

It is important to note that although testosterone is an androgen, it is also manufactured (secreted) and found in much lower levels in females. Elevated levels of testosterone in females may lead to the development of male-like secondary sexual characteristics (masculinization).

Teststosterone and other steroid hormones (e.g., estrogen, cortisone, etc) are derived from cholesterol. At the molecular level, steroid hormones such as testosterone consist of four interlocking rings of carbon atoms (three of the rings contain six carbon atoms and the fourth ring contains five carbon atoms) that with hydrogen atoms bonded to the ringed carbon atoms, form a hydrogenated cyclopentophenanthrene-ring. Testosterone is produced in the interstitial cells of Leydig that lie between the seminiferous tubules.

After secretion by the testes, testosterone enters the blood stream to circulate throughout the body. As the testosterone circulates, it binds to specific receptors on target tissues. Within the cell, testosterone is converted into dihydrotestosterone--and it is in this form that testosterone enters the cellular metabolism. Dihydrotestosterone binds with other proteins to form a complex that is capable of entering the nucleus and stimulating DNA transcription.

Testosterone that fails to bind to tissues degrades into inactive substances that are removed by excretion. Most of these biochemical breakdowns occur in the liver.

Although there are other male sex hormones produced by the testes, testosterone is by far the most abundant hormone produced. Elevated testosterone levels occur at two very different and distinct times in normal male development. Increased testosterone secretion occurs in newborn male infants and with the onset of puberty.

In the embryo, testicular tissue, stimulated by placental gonadotropin hormones, produced the higher levels of testosterone needed for normal male development during organogenisis (formation of organs). The synthesis of testosterone begins during the second month of gestation and acts principally to influence the differential development of the genital ridge in males. Physiological tests on other animals demonstrate that the introduction of testosterone during this stage of development can lead to the formation of male genitalia even if the developing embryo is genetically female. Testosterone stimulates the genital ridge form a penis, scrotum, seminal vesicles, and prostate gland, and suppresses the development of a female clitoris and vagina.

In the newborn male, testosterone also induces the descent of the testis into the scrotum through the inguinal canals.

Luteinizing hormone (LH) produced by the pituitary gland stimulates testosterone production in the pubescent and adult male. LH acts in coordination with follicle stimulating hormone (FSH) to promote the production of sperm cells (spermatogenesis). In the adult, increased testosterone secretion causes enlargement of the penis, scrotum, and testes and induce the development of secondary sexual characteristics, including the development of body hair. Interestingly, testosterone inhibits the growth of hair on the top of the head and--along with a genetic predisposition to baldness--may eventually cause baldness. Testosterone also causes a thickening of skin and of the laryngeal mucosa and larynx that produce the deeper male voice.

In addition to promoting a higher metabolic rate, testosterone cases increase nitrogen retention and protein synthesis associated with muscle growth. In addition, testosterone acts to promote calcium retention and bone matrix development that produces the thickened male skeletal structure.