Testosterone
Many benefits can be derived by testosterone replacement therapy, both in the short term for the eradication of symptoms of androgen deficiency and in the long term for the prevention of osteoporosis and for preventing and treating heart and circulatory problems.  Testosterone is not, however, effective in erectile dysfunction.  Androgen replacement therapy should, ideally, provide physiological serum testosterone levels, as well as dihydrotestosterone and estradiol levels while correcting the clinical symptoms of androgen deficiency in hypogonadal men.

Testosterone Replacement
Testosterone is the androgenic hormone primarily responsible for normal growth and development of male sex and reproductive organs, including the penis, testicles, scrotum, prostate, and seminal vesicles. It facilitates the development of secondary male sex characteristics such as musculature, bone mass, fat distribution, hair patterns, laryngeal enlargement, and vocal chord thickening. Additionally, normal testosterone levels maintain energy level, healthy mood, fertility, and sexual desire.

As men age, the decline in male sex hormones, called androgens, result in decreased muscular strength, energy, libido and an increased risk of erectile dysfunction and sexual performance.

When there is less testosterone available to do its work, the testosterone target-organ response decreases, bringing about many changes.

There is great variability in testosterone levels among healthy men so not all will experience the same changes to the same extent. But typical responses to low bio-available testosterone levels include:

• Low sex drive 
• Emotional, psychological and behavioral changes
• Decreased muscle mass 
• Loss of muscle strength 
• Increased upper and central body fat 
• Osteoporosis or weak bones and back pain
• Cardiovascular risk

Signs and Symptoms
Although diagnosis can only be made by a licensed physician, men are generally reluctant or unwilling to acknowledge that the syndrome has crept up on them.  The symptoms are not as overwhelming as the dramatic changes women experience and it may not affect all men; however, about 40% of men in the 40s, 50s, and 60s will experience some of the symptoms, including lethargy, depression, irritability, mood swings, and erectile dysfunction.

Testosterone levels begin to decrease for a number of reasons, including (1) the Leydig cells begin to decrease in number and function, (2) a sex hormone binding globulin (SHBG) increase with age, resulting in greater binding of testosterone with less free testosterone.  There is a higher relative amount of estradiol with less testosterone being produced.

Visceral fat increase has been associated with an increased vascular risk. Body fat in men increases from 18% to 36% between 18 and 85 years of age, with the largest increases in intraabdominal fat. Also, by age 70, an average man has about 26 pounds less lean body mass than at age 25.

Low testosterone can lead to osteoporosis in elderly men. This tends to happen later in men than in women because of the general tendency for men's bones to be thicker and denser than women's. In hypogonadal men, bone mineral density tends to increase with testosterone treatment. In men, there may be an increase in circulating estrogen levels; common causes of estrogen increase during mid-life include age-related increases in aromatase activity, alteration in liver function, zinc deficiency, obesity, overuse of alcohol, drug-induced estrogen imbalance and ingestion of estrogen-enhancing food or environmental substances.  Also, fatty tissue contains more aromatase activity as compared with lean tissue resulting in more testosterone being converted to estradiol. Vitamin C deficiency is associated with high levels of aromatase activity whereas zinc inhibits aromatase activity.

Low testosterone levels may tend towards depression; the decrease in sexual function may also lead to depression, irritability, and mood swings...this depression further leading to decreased sexual function.

Who qualifies for testosterone therapy programs.
The decision to use HRT is an individual one, based on the individual's particular risks. The goals of natural HRT are to:

• Alleviate the symptoms caused by the natural decrease in production
  of hormones by the body.Replace the hormones to the extent to provide positive benefits.Bring the body back to normal hormonal balance.
• Imitate the body's natural processes as much as possible.

Natural testosterone replacement is central to the treatment of all aspects of "male menopause". Testosterone supplementation has resulted in decreasing body fat mass from 6.4% to 1.4% and increases in lean mass from 3.2% to 5%.  Increases in strength (grip strength) also are reported.