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.