AMA #28: All things testosterone and testosterone replacement therapy

This is a very important point to understand clinically. If a person is supplementing with testosterone, it is usually very obvious to tell this from their blood work because they have unmeasurable levels of LH and FSH and usually high levels of testosterone. Now, at some point, this becomes a permanent issue. In other words, at some point, if a person is taking exogenous testosterone for long enough, their body will lose the ability to make its own.

When testosterone is low, the feedback cycle to the brain, ultimately, is to ramp up the secretion of LH [luteinizing hormone] and FSH [follicle-stimulating hormone]. Conversely, when testosterone is high, the signal that's sent back is to inhibit the production of these things.

It's only the unbound portion of testosterone that is able to actually exert the biological influence. So, we pay very special attention to how much testosterone is 'free'—and free is defined as the testosterone that is neither bound to SHBG or albumin.

And that's effectively what testosterone is doing—it is up or downregulating genes that are responsible for a number of things, but the most obvious of these are kind of the anabolic or growth characteristics.