Testosterone is a steroid hormone from the androgen group. Testosterone is primarily secreted in the testes of males and the ovaries of females, although small amounts are also secreted by the adrenal glands. It is the principal male sex hormone and an anabolic steroid. In both males and females, it plays key roles in health and well-being. Examples include enhanced libido, energy, immune function, and protection against osteoporosis. On average, an adult human male body produces about eight to ten times more testosterone than an adult female body
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 do not use testosterone as much) 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.
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.
A lack of testosterone is strongly linked to depression and anxiety syndromes, when a lack of testosterone and a hypogondal state are the cause of such problems restortion of eugondal status by appropriate testosterone replacement often helps reduce or resolve such conditions.
Contrary to what has been postulated in outdated studies and by certain sections of the media, agressive behaviour is not typically seen in hypogondal men who have their testosterone replaced adequately to the eugonadal/normal range. In fact aggressive behaviour has 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).
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