SHBG's Impact on Testosterone Bioactivity

Mechanisms of SHBG's Effect on Testosterone

• SHBG binds and transports testosterone in plasma, with only a small fraction of testosterone remaining in the unbound (free) form 1, 2
• Free testosterone (FT) is the biologically active form of testosterone that is unbound to SHBG or albumin, making it a more sensitive indicator of hyperandrogenemia than total testosterone 1, 2

Clinical Measurement and Significance

• Calculated free testosterone (cFT) can be estimated from total testosterone and SHBG measurements using formulas like the Vermeulen equation 1, 2
• Free Androgen Index (FAI), derived from the total testosterone to SHBG ratio, is a simple tool for assessing bioavailable testosterone but can be inaccurate if SHBG concentration is low 1, 2

Factors Affecting SHBG Levels

• SHBG levels can be elevated due to medications, aging, smoking, hyperthyroidism, hepatic disease, and HIV/AIDS 3

Clinical Implications

• Elevated SHBG can contribute to symptoms of hypogonadism even with normal total testosterone levels 3
• Focusing only on total testosterone can be misleading; free or bioavailable testosterone measurements provide better insight into androgen status 3