Sex Hormone Binding Globulin (SHBG)

The Sex Hormone Binding Globulin (SHBG) test, also known as sex steroid–binding globulin or testosterone–estradiol–binding globulin, is ordered when total testosterone does not fit the clinical picture. It supports the evaluation of male hypogonadism, infertility, decreased libido, and erectile dysfunction, and it assists in assessing hyperandrogenism in women, including suspected polycystic ovary syndrome. SHBG is often measured with total and free testosterone, albumin, prolactin, estradiol, and luteinizing hormone, and is used to calculate the free androgen index (total testosterone/SHBG).

SHBG is synthesized by the liver and binds testosterone, dihydrotestosterone, and estradiol, thereby governing their transport and tissue availability. In adults, about 40% to 60% of circulating testosterone is tightly bound to SHBG, most of the remainder is loosely bound to albumin, and approximately 2% is unbound; only the albumin-bound and free fractions are considered bioavailable. Total testosterone reflects all binding states and does not differentiate SHBG-bound from unbound hormone. When SHBG is abnormally high or low, the total testosterone concentration may over- or underestimate the physiologically available androgen pool. SHBG levels vary by age and sex, tend to rise with aging in men, and decline after menopause in women; thyroid function, liver disease, and body mass also influence SHBG concentrations. Accordingly, SHBG measurement is used to place total testosterone results in physiologic context and to support calculation of the free androgen index.