Sex hormone binding globulin (SHBG) is a glycoprotein produced by the liver that binds tightly to sex hormones — primarily testosterone and, to a lesser extent, oestradiol — and transports them through the bloodstream. Testosterone bound to SHBG is biologically inactive and cannot enter cells or exert hormonal effects.

SHBG levels profoundly influence the amount of free and bioavailable testosterone in the blood. High SHBG reduces the active hormone fraction and can cause symptoms of low testosterone even when total testosterone appears normal. Conversely, low SHBG increases free testosterone availability. SHBG is elevated by oestrogen, thyroid hormone, and ageing, and suppressed by insulin resistance, obesity, hypothyroidism, and anabolic steroids.