After age 30, most men begin to experience a natural and gradual decline in testosterone levels, which can result in reduced sex drive and performance, a loss of muscle tone and definition, very rapid weight gain especially fat around the belly, severe mood swings, depression, insomnia and sheer exhaustion. Most of the testosterone found in men is bound to the Sex Hormone Binding Globulin (SHBG) and Albumin. Only 2 to 3% of the testosterone present in a man's body is typically in a free or bio-available state. If a man's testosterone levels are low, he may require TRT or Testosterone Replacement Therapy to boost low levels back up into the normal range.

We at Modern Therapy have Testosterone Medical Specialists and Hormone Replacement Physicians providing Andropause Treatment for Men with Low T or Low Testosterone Levels. Visit our Testosterone Therapy page to learn more about how androgens work in the body and how increasing testosterone can provide potential health benefits to men and women - Testosterone Treatments for Men with Low T.

Like other steroid hormones, testosterone is derived from cholesterol. The largest amounts of testosterone are produced by the testes in men. It is also synthesized in far smaller quantities in women by the thecal cells of the ovaries, by the placenta, as well as by the zona reticularis of the adrenal cortex in both sexes.

In the testes, testosterone is produced by the Leydig cells. The male generative glands also contain Sertoli cells which require testosterone for spermatogenesis. Like most hormones, testosterone is supplied to target tissues in the blood where much of it is transported bound to a specific plasma protein, sex hormone binding globulin (SHBG).

As testosterone affects the entire body (often by enlarging; men have bigger hearts, lungs, liver, etc.), the brain is also affected by this "sexual" advancement; the enzyme aromatase converts testosterone into estradiol that is responsible for masculinization of the brain in a male fetus.

There are some differences in a male and female brain (the result of different testosterone levels). A clear difference is the size: the male human brain is, on average, larger; however, in females (who generally do not have as high testosterone levels) the corpus callosum is proportionally larger. This means that the effect of testosterone is a greater overall brain volume, but a decreased connection between the hemispheres.

A study conducted in 1996 found no effects on mood or behavior from the administration of supraphysiologic doses of Testosterone for 10 weeks to healthy men.

The literature suggests that attention, memory, and spatial ability are key cognitive functions affected by testosterone in humans, though the literature is rather sparse. Preliminary evidence suggests that low testosterone levels may be a risk factor for cognitive decline and possibly for dementia of the Alzheimer’s type, a key argument in Life Extension Medicine for the use of testosterone in anti-aging therapies. Much of the literature, however, suggests a curvilinear or even quadratic relationship between spatial performance and circulating testosterone, where both hypo- and hypersecretion of circulating androgens have negative effects on cognition and cognitively-modulated aggressivity, as detailed above.

Contrary to what has been postulated in outdated studies and by certain sections of the media, aggressive behaviour is not typically seen in hypogonadal men who have their testosterone replaced adequately to the eugonadal/normal range. In fact aggressive behaviour has been associated with hypogonadism and low testosterone levels and it would seem as though supraphysiological and low levels of testosterone and hypogonadism cause mood disorders and aggressive behaviour, with eugondal/normal testosterone levels being important for mental well-being. Testosterone depletion is a normal consequence of aging in men. One consequence of this is an increased risk for the development of Alzheimer’s Disease (Pike et al, 2006, Rosario 2004).