The cause and effect is different.
High SHBG results in any given amount of free test yielding a higher total testosterone level than one would, lacking the SHBG knowledge, expect.
While true that if we know the total T and we know SHBG is high then we know that free T is lower than otherwise would be expected for that total T, it is not the case that the SHBG is a causative factor of the free T being low.
An analogy might help illustrate. Suppose we measure “total body testosterone” by some imaginary CAT-scan-like method.
Now a say moderately fat guy, for same blood levels of testosterone, will have way more total body testosterone due to the amount dissolved in fat.
But if the moderately fat guy asks, “My total body testosterone is high but my free T is low,” the reason for his low free T is not that he is fat, and the way for his free T to improve is not that he needs to lose fat.
Because of the nature of chemical equilibria, the phenomenon does exist that free T will be “lower than expected” for given total body testosterone when fat, but that does not mean that storage of testosterone in body fat is the cause of low free T. Rather, the free T is what the free T is, and the high total value is because there is a lot of fat. (
Yes, I know that higher aromatase activity associated with fatness could yield lower free T particularly when frankly obese but that is a different mechanism not analogous to our problem here and is not intended as part of the illustration. No illustration is perfect, or few are anyway.)
Ditto for high SHBG.
The low free T is caused by low LH or poor testicular sensitivity.