Testosterone is the most important hormone determining sexual characteristics in males. Testosterone determines whether a baby will develop into a girl or a boy. In males, testosterone is primarily produced in the testes. In females, it is primarily produced in the ovaries. Small amounts of the hormone are produced in the adrenal glands in both sexes.
Structure and Physical Characteristics
Testosterone is comprised of carbon, hydrogen and oxygen. The molecular weight of testosterone is 288.4 and the chemical name of the hormone is 17beta-Hydroxyandrost-4-en-3-one. It takes the form of solid crystals or crystalline powder and has a white or slightly creamy-white color.
Testosterone is the major sex hormone in males and plays a number of important roles, such as:
- The development of the penis and testes
- The deepening of the voice during puberty
- The appearance of facial and pubic hair starting at puberty; later in life, it may play a role in balding
- Muscle size and strength
- Bone growth and strength
- Sex drive (libido)
- Sperm production
The earliest function of testosterone is to direct prenatal development. Male fetuses start producing testosterone early in gestation; it's made by the testes, and circulates throughout the body. In her book "Human Physiology," Dr. Lauralee Sherwood explains that early in development, both male and female fetuses look the same. Once the testes of a male fetus begin to synthesize testosterone, however, the female reproductive ducts die, while male reproductive ducts develop. Formerly undifferentiated external genitals become a penis and scrotum. The testosterone even has an effect on the brain of an unborn male baby; it causes the communication center of the brain to atrophy slightly relative to that of a female baby, and increases the size and activity of the aggression center.
During puberty, testosterone has further physical effects upon boys. It causes the skin to produce more sweat glands and grow more body hair, explains Dr. Gary Thibodeau in his book, "Anatomy and Physiology." It increases bone density and musculature, giving men heavier, thicker bodies than women. It thickens the tissue of the larynx, or voice box, leading to deepening of a man's voice. It also causes the testes to produce sperm, making a male reproductively capable. In short, during puberty, testosterone leads to development of what are called the masculine secondary sex characteristics, where primary sex characteristics are the testes and sex organs.
Adolescent boys with too little testosterone may not experience normal masculinization. For example, the genitals may not enlarge, facial and body hair may be scant and the voice may not deepen normally.
Testosterone may also help maintain normal mood. There may be other important functions of this hormone that have not yet been discovered.
Signals sent from the brain to the pituitary gland at the base of the brain control the production of testosterone in men. The pituitary gland then relays signals to the testes to produce testosterone. A "feedback loop" closely regulates the amount of hormone in the blood. When testosterone levels rise too high, the brain sends signals to the pituitary to reduce production.
If you thought testosterone was only important in men, you'd be mistaken. Testosterone is produced in the ovaries and adrenal gland. It's one of several androgens (male sex hormones) in females. These hormones are thought to have important effects on:
- Ovarian function
- Bone strength
- Sexual behavior, including normal libido (although evidence is not conclusive)
The proper balance between testosterone (along with other androgens) and estrogen is important for the ovaries to work normally. While the specifics are uncertain, it's possible that androgens also play an important role in normal brain function (including mood, sex drive and cognitive function).
Effects of Excessive Testosterone
Having excessive naturally occurring testosterone is not a common problem among men. Most of what is known about the effects of excessive testosterone levels comes from studies conducted on those taking anabolic steroids or otherwise artificially increasing their testosterone levels. High levels of testosterone can cause a number of problems in men. These include high blood pressure, high cholesterol, elevated risk of blood clots, elevated risk of heart attack, damage to the heart muscle, enlarged prostate and liver disease. In addition, excessive testosterone can lead to low sperm count, shrinking of the testicles and even impotence. Among women, excessive testosterone may lead to many of the aforementioned problems as well as a decrease in breast size and deepening of the voice.
Effects of Low Testosterone
There are a number of problems associated with having testosterone levels that are too low. In men, problems may include loss of muscle mass, brittle bones (along with an increased risk of fracture), irritability, poor concentration and depression. In addition, men with low testosterone levels may suffer from low libido, reduced sperm count, infertility and impotence. Among women, low testosterone may lead to lower bone strength, low libido and depression.
Diseases and Conditions that Decrease Testosterone
In men, testosterone levels may be reduced by a direct injury to the testes, radiation treatment and infection or tumors of the testes. In addition, HIV/AIDS can reduce testosterone levels in men. In women, testosterone deficiency may be caused by diseases of the pituitary, hypothalamus or adrenal glands. In addition, estrogen therapy will reduce the amount of free and active testosterone in a woman's body.
Aging and Testosterone Levels
The aging process is characterized by a decline of most physiological functions. Among these, the decline in endocrine functions plays an important role in the symptomatology of the aging process. In contrast to women, who experience a rather abrupt termination of the ovarian cyclic hormonal activity, in men both endocrine (testosterone) and exocrine (spermatogenesis) testicular functions are preserved until very old age. Hence, the male equivalent of the menopause: the andropause, does not really exist. Nevertheless, both endocrine and exocrine function decline with age. Whereas it has long been debated whether plasma testosterone concentration decreases with age in healthy men, the occurrence of an age-associated decrease in bioactive testosterone concentration is no longer disputed.
The normal range of testosterone is reported as 350- 1200ng/dl. Studies in the 1940's showed the average testosterone level to be at 700 ng/dl, 300 ng/dl higher than for men today. In the past, a drop in testosterone levels to 250 ng/dl was rarely reported before men were 80 years of age. Yet today, it is not an uncommon value for middle aged men!
Testosterone levels are highest in the early twenties. The decrease in serum levels is now occurring at an even earlier age. Up to 50% of all men at 40 now have testosterone levels below what was considered the normal range of 450 ng/dl. Recent studies imply that the pesticides and preservatives in foods and the hormone pellets to fatten up cattle, pork and chicken act as "hormonal disruptors." Based on the low sperm counts, infertility, obesity and low serum testosterone seen in younger men, this assumption might be true.
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