Sex hormone binding globulin (SHBG) is a glycoprotein composed of 373 amino acid residues and three carbohydrate side chains. SHBG has been known by many other names including Testosterone-estradiol Binding Globulin (TeBG), Sex steroid Binding Protein (sBP) and Sex Steroid Binding Globulin (SSBG). One of the main properties of SHBG is its high affinity for steroids, especially the C18, C19 and 17α-hydroxyl groups. The binding of steroids to SHBG is temperature and pH dependent. The three steroids that have a high avidity for SHBG are Dihydrotestosterone, Testosterone and Estradiol. Very small amounts of these steroids are free in biological fluid; the majority are bound to SHBG and albumin. These two fractions, that is, free and bound exist in a state of dynamic equilibrium. When the level of SHBG concentration changes, a remarkable change occurs in both albumin-bound hormone and also in the free fraction. Throughout life SHBG increases until the eighties in both sexes. During the menstrual cycle SHBG does not seem to vary appreciably, however, according to some authors the concentration of SHBG is elevated in the luteal phase. During pregnancy the level of SHBG rises rapidly until about the 30th week of gestation.