SHBG and Free Testosterone Explained

Sex hormone-binding globulin (SHBG) is a protein that binds to testosterone in the bloodstream. Testosterone bound to SHBG is not biologically available for the body to use — only the unbound portion is active.

Total testosterone measures everything in the blood — bound and unbound together. Free testosterone measures only the unbound, biologically active portion. A man can have a normal total testosterone reading while his free testosterone tells a different story, depending on his SHBG level.

SHBG levels themselves vary with age, obesity, thyroid function and liver health, among other factors. Someone with high SHBG can have low free testosterone despite a normal-looking total figure, and vice versa — which is why SHBG is measured alongside testosterone, not assumed to be constant.

Relying on total testosterone alone can miss a genuine deficiency, or suggest one that isn't really there. This is part of why a full hormonal assessment looks at total and free testosterone together with SHBG, rather than treating a single number as the whole story.