What is SHBG (sex hormone binding globulin)?
SHBG is a protein produced by the liver that binds to testosterone in the blood, making it unavailable for tissue use and affecting how testosterone levels are measured and interpreted.
Sex hormone binding globulin (SHBG) is a protein made by the liver that attaches to testosterone molecules once they enter the bloodstream. When SHBG binds testosterone, that testosterone becomes inactive and unavailable to cells and tissues that need it. Only unbound testosterone, called free testosterone, can actually enter cells and produce effects in the body.
This distinction matters significantly during testosterone replacement therapy. A patient's total testosterone level includes both SHBG-bound testosterone and free testosterone. However, only the free portion does useful work. Someone with a high total testosterone count may have low free testosterone if SHBG levels are elevated, meaning they experience few actual benefits from the hormone despite what the total number suggests.
SHBG levels vary based on age, body weight, liver function, and certain medications. Higher SHBG reduces bioavailable testosterone, while lower SHBG increases it. This is why doctors who specialize in testosterone therapy look beyond total testosterone numbers alone. Interpretation requires understanding individual SHBG levels, which is why working with endocrinologists and hormone specialists ensures accurate diagnosis and appropriate dosing decisions rather than relying on total testosterone readings in isolation.