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Sex hormone binding globulin (SHBG) is a protein that is produced by the liver and binds tightly to the hormones testosterone, dihydrotestosterone (DHT), and estradiol (an estrogen). In this bound state, SHBG transports these hormones in the blood as biologically inactive forms. This test measures the level of SHBG in the blood and is most often used to help evaluate for testosterone deficiency or excess. In men, about 45% to 65% of testosterone in blood is normally bound to SHBG, with the remainder weakly and reversibly bound to albumin (the main protein in the blood). Only about 2% to 3% of testosterone is immediately available to the tissues as free testosterone, but testosterone that is weakly bound to albumin is also bioavailable and can be readily taken up by the body’s tissues.