Sex hormone-binding globulin (SHBG) is a glycoprotein responsible for blood transport of testosterone and estradiol. SHBG is synthesized in the liver and has a high binding affinity for 17-hydroxysteroid hormones.1,2 Less than 2% of biologically active steroids are free in the circulation with the remainder being bound mostly to SHBG and albumin. SHBG has a high binding affinity to the 17-hydroxysteroid hormones while albumin has a low binding affinity.3 Initially, the free portion or unbound hormone fraction was believed to be the only biologically active form. It is now recognized that the portion of hormone that is weakly bound to albumin is also available to the tissues. The free hormone plus the albumin bound portion of hormones represents the “bioavailable” hormone.