Why It Matters
Sex hormone-binding globulin (SHBG) is a glycoprotein produced primarily by the liver that binds to sex hormones, particularly testosterone and estradiol, rendering them biologically inactive. Only the unbound (free) fraction of these hormones can interact with tissue receptors and exert physiological effects. SHBG therefore acts as a regulatory buffer on androgenic and estrogenic activity throughout the body (source).
In athletes, SHBG is critical for interpreting total testosterone correctly. An athlete may have normal or even high total testosterone but functionally low free testosterone if SHBG is elevated, blunting the anabolic signal available to muscle and bone tissue (source).
Low SHBG: Insulin Resistance and Metabolic Stress
Low SHBG is strongly associated with hyperinsulinemia and insulin resistance, as insulin suppresses hepatic SHBG production. In athletes, low SHBG may indicate excessive carbohydrate intake relative to metabolic need, visceral fat accumulation, or early-stage metabolic syndrome. It can also appear during periods of high anabolic hormone output (e.g., during post-pubescent male athletes in strength-focused training phases) (source).
Low SHBG increases free testosterone and estradiol, which may appear beneficial short-term but can disrupt hormonal feedback loops and increase estrogen-related side effects in male athletes (e.g., gynecomastia risk) or amplify androgenic effects in females.
High SHBG: Hormonal Binding and Reduced Bioavailability
Elevated SHBG is common in endurance athletes, older athletes, and those experiencing caloric restriction or thyroid dysfunction. When SHBG is high, a greater fraction of testosterone is bound and unavailable, reducing the anabolic stimulus to muscle tissue even if total testosterone is within range. This is particularly relevant in masters athletes and female endurance athletes where absolute testosterone levels are already lower (source).
Other causes of elevated SHBG include liver disease (increased hepatic synthesis), hyperthyroidism, estrogen therapy, and aging. Oral contraceptive use in female athletes substantially raises SHBG and must be considered when interpreting free testosterone.
Coaching Application
Always calculate or directly measure free testosterone when evaluating androgenic status. Free testosterone = (total T × 0.0226) / (1 + SHBG × 5.97 × 10⁻⁹) is a reasonable approximation. An athlete with suppressed free testosterone due to high SHBG despite normal total testosterone may benefit from interventions targeting SHBG reduction: adequate caloric intake, optimized zinc and magnesium status, and load management to reduce cortisol (which also raises SHBG).
