Free Testosterone
Only free testosterone is active and can bind to androgen receptors. Only free testosterone can engage in chemical reactions, specifically converting into dihydrotestosterone.
The level of free testosterone increases during puberty (in men, only at later stages), and without such an increase, the formation of secondary sexual characteristics would be impossible.
As the body ages, it is primarily the level of free testosterone, not total testosterone, that declines first. This ultimately leads to a decrease in libido, a reduction in overall vitality, memory problems, cognitive issues, and more.
Under normal conditions, about two percent of the total testosterone in a man's body should be free testosterone. However, a very high level of free testosterone (if maintained over a long period) is also detrimental.
Testosterone Bound to SHBG
A reasonable question: why bind sex hormones at all? The answer is quite simple: to transport them to their destination – initially into the bloodstream.
The fact is that unbound hormone very quickly "becomes inactive" and leaves the body, often without having the chance to do anything useful.
With some degree of approximation, we can say that substances that bind sex hormones act as transporters, meaning they are absolutely necessary for the transport of testosterone, particularly to muscles.
Who plays the role of these transporters by binding testosterone? Primarily, SHBG – which stands for Sex Hormone-Binding Globulin. SHBG is mainly produced in the liver and binds about 60% of all available testosterone.
The "sex hormone - SHBG" complex is remarkably stable, but sometimes it breaks down, allowing the sex hormone to become free and start acting "like an adult."
However, if a free SHBG molecule is nearby, it will most likely capture the sex hormone molecule before it can do anything useful.
Testosterone Bound to Albumin
Testosterone can bind not only to SHBG but also to albumin present in the blood, which is also mainly produced in the liver.
This bond is much weaker and breaks down relatively easily. Because of the weak bond with albumin, such testosterone is sometimes called conditionally free. About 40% of our testosterone is in this form.
On one hand, the weak bond between testosterone and albumin is a plus: more testosterone can "break free." On the other hand, it's a minus: significantly fewer sex hormones are delivered to their destination by albumin.
On average, according to one old but very reliable study, the latter amount is about 55% of all albumin-bound testosterone.
However, the fact that about 22% of conditionally free testosterone (40% multiplied by 0.55) is in close proximity to the target doesn't mean much. According to studies, at any given time, only one to four percent of testosterone is in a free – and thus maximally active – state.
However, the more albumin (and the more testosterone bound to albumin) in our body, the more testosterone will eventually be "free": the ratio of free testosterone to albumin-bound testosterone remains relatively constant.
Therefore, one way to increase the level of free testosterone is to stimulate albumin production.
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