SHBG

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. It binds tightly to testosterone, dihydrotestosterone (DHT), and oestradiol (an oestrogen) and transports them in the blood in a metabolically inactive form. The amount of SHBG in circulation is affected by age and sex, by decreased or increased testosterone or oestrogen production, and can be affected by certain diseases and conditions such as liver disease, hyperthyroidism or hypothyroidism, and obesity.

Changes in SHBG concentrations can in turn affect the amount of testosterone that is available to be used by the body’s tissues. Normally, about 40% to 60% of testosterone is bound to SHBG, and most of the rest is weakly and reversibly bound to albumin (another protein). Only about 2% is immediately available to the tissues as free testosterone.

A total testosterone does not distinguish between bound and unbound testosterone; it determines the overall quantity of testosterone. In many cases, this is sufficient to evaluate excessive or deficient testosterone production; but, if a patient’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 a patient’s tissues.

A blood sample is obtained by inserting a needle into a vein in the arm.