Measuring Testosterone levels is not as straightforward as it can appear. There are five distinct categories that define total Testosterone markers and levels: total, free, bound (Albumin + SHBG), and bioavailable (free + Albumin).
is the absolute total testosterone circulating in your bloodstream. A "Total" Testosterone lab test measures free testosterone as well as what is bound to the proteins albumin and sex hormone binding globulin (SHBG).
Free Unbound Testosterone
Considered the most abundant protein in your blood; around 50% of Testosterone in your body is attached to Albumin. When testosterone binds to Albumin, it is considered bioavailable but is not as readily active as unbound Testosterone would be. This might sound totally confusing but know that it’s not what’s considered free but it accounts for what’s bioavailable in your bloodstream, which includes your free/unbound amounts.
Sex Hormone Binding Globulin is a protein that’s made in the liver and attaches itstelf to sex hormones, primarily Testosterone, and removes it from direct utilization in the body. One bound, it inhibits Testosterone really from doing anything of use in the body, hence why SHBG bound T is unavailable to your body. High levels of SHBG can throw off what’s actually available (free) for your bodies to use, so it’s important to differentiate this from what’s bioavailable.