Sex Hormone-Binding Globulin
Sex hormone-binding globulin (SHBG) is a glycoprotein that binds to sex hormones, specifically testosterone and estradiol. These sex hormones circulate in the bloodstream bound mostly to SHBG and, to some degree, albumin. Only a small fraction is unbound, or "free," and thus biologically active and able to enter a cell to activate its receptor. Bioavailability of sex hormones is influenced by the level of SHBG in this manner. SHBG levels are controlled by a delicate balance of enhancing and inhibiting factors. Its level is decreased by high levels of insulin and insulin-like growth factor-I (IGF-I). Also, high androgen levels decrease SHBG, while high estrogen and thyroxine levels increase it. Conditions with low SHBG include polycystic ovary syndrome, diabetes, and hypothyroidism. Conditions with high SHBG include pregnancy, hyperthyroidism, and anorexia nervosa.
Swiss-Prot Accession Number: P04278
