Sex Hormone Binding
The sex hormone binding globulin (SHBG) test measures the concentration of SHBG in the blood. SHBG is a protein that is produced by the liver and binds tightly to testosterone, dihydrotestosterone (DHT), and estradiol (an estrogen). In this bound state, it transports them in the blood as an inactive form. The amount of SHBG in circulation is affected by age and sex, by decreased or increased testosterone or estrogen production and can be affected by certain diseases and conditions such as liver disease, hyperthyroidism or hypothyroidism, and obesity. Changes in SHBG levels can affect the amount of testosterone that is available to be used by the body's tissues.
A total testosterone test does not distinguish between bound and unbound testosterone but determines the overall quantity of testosterone. If a person's SHBG level is not normal, then the total testosterone may not be an accurate representation of the amount of testosterone that is available to the person's tissues.