A "SHBG" (Sex Hormone Binding Globulin) hormone assay is a blood test that measures the level of a protein called Sex Hormone Binding Globulin in your blood, which plays a key role in regulating the amount of active sex hormones (like testosterone and estrogen) available to your body tissues by binding to them and essentially "transporting" them throughout the bloodstream; essentially, higher SHBG levels mean less free hormone is available for use by the body.