Testosterone is a steroid hormone present in both men and women (men have much more). Some testosterone floats about the body in the blood without being attached to anything else. This is called 'free' testosterone.
The rest of the testosterone is attached (called bound), some to a protein called sex hormone binding globulin (SHBG ) and some to a protein called albumin.
'Total' testosterone is the sum of all the testosterone in the blood, no matter whether it is f'free' or bound.
Free testosterone is just the free stuff floating by itself (not bound). Only a small precentage of testosterone is free.
For years, doctos thought only free testosterone was biologically active, meaning it was thought that the free testosterone was doing all the things testosterone is supposed to do, while the bound testosterone attached to SHBG or albumin and doesn't do anything. Some researchers disagree with this and think that the testosterone bound to albumin is also active. The jury is still out.
If someone has normal levels of total testosterone, but a low SHBG, then it's likely there's more free testosterone. Alternatively, someone who has high SHBG would likely have a lower amount of free testosterone, since the SHBG will bind to more of it.
That's why it's useful to know free testosterone levels, rather than just total testosterone.
