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A blood protein that binds sex hormones; high SHBG lowers usable (free) testosterone.
Sex hormone-binding globulin is a liver-made protein that binds testosterone and estradiol in the bloodstream, regulating how much is free and active. High SHBG (from aging, thyroid issues, or some medications) reduces free testosterone even when total testosterone reads normal; low SHBG (often with obesity or insulin resistance) does the reverse. It is why total testosterone alone can mislead.