Does high SHBG with low total testosterone cause a disproportionate drop in free testosterone?

Mechanistic explanations

• High-affinity binding kinetics: Testosterone in circulation exists in equilibrium: approximately 40% to 60% is bound with high affinity to sex hormone-binding globulin (SHBG), about 40% to 50% is loosely bound to albumin, and only 1% to 3% remains unbound or "free."
• Disproportionate free fraction decline: Because SHBG has a very high binding affinity for testosterone, an elevation in SHBG levels forces a larger proportion of the available testosterone into a bound state. When total testosterone levels are simultaneously low, the binding dynamics dictate that a disproportionately greater relative percentage of the remaining testosterone is bound to SHBG, causing the biologically active free testosterone fraction to fall sharply and disproportionately compared to total testosterone.
• Impact of aging: In women, aging and the menopausal transition are associated with changes in binding proteins. An age-related increase in SHBG further exacerbates this imbalance, driving down the free testosterone index even when total testosterone levels appear stable or only moderately reduced.

Clinical implications and diagnostics

• Inadequacy of total testosterone screening: Because of these binding kinetics, measuring total testosterone alone can be highly misleading. A patient can have a total testosterone level within the normal reference range but still suffer from tissue-level androgen deficiency due to severely depressed free testosterone levels driven by high SHBG.
• Mathematical estimation: To accurately assess androgen status, clinicians utilize established algorithms (such as the Vermeulen or Södergård equations) that calculate free testosterone by factoring in total testosterone, SHBG, and albumin levels, rather than relying solely on total testosterone assays.

Bottom line

When SHBG is high and total testosterone is low, the high affinity of SHBG causes a disproportionate drop in free testosterone. Consequently, total testosterone levels can mask clinical androgen deficiency, making the assessment of free testosterone or SHBG essential for accurate clinical evaluation.