Sex hormone binding globulin (SHBG) is a glycoprotein synthesized by the liver. Circulating androgen and estrogen concentrations influence SHBG synthesis. Elevated testosterone, for example, causes SHBG synthesis to decrease, whereas high estrogen stimulates SHBG production. The regulation of SHBG synthesis, combined with SHBG's higher affinity for testosterone, impacts bioavailable testosterone levels. SHBG binds up to 98 percent of the steroid hormones in the blood including 5a-dihydrotestosterone (DHT), testosterone and androstenediol with particularly high affinity, and estradiol and estrone with slightly lower affinity.