Androgen Bioavailability Metrics are the specific biochemical measurements used in clinical endocrinology to quantify the fraction of androgens, primarily testosterone, that is physiologically active and available to bind to target tissue receptors. The most relevant metrics include Free Testosterone, which is unbound to carrier proteins, and Bioavailable Testosterone, which comprises both free testosterone and the fraction loosely bound to albumin. Accurate assessment provides a superior clinical picture compared to Total Testosterone alone, especially when Sex Hormone-Binding Globulin (SHBG) levels are abnormal. This measurement is crucial for correlating laboratory results with clinical symptoms of deficiency or excess.
Origin
The term combines “Androgen” (Greek andros ‘man’ + -gen ‘producing’), “Bioavailability” (referring to the proportion of a substance that enters the circulation and has an active effect), and “Metrics” (from Greek metron ‘measure’). This terminology reflects the scientific shift toward measuring the functional, active component of a hormone. The necessity for these metrics arose from the realization that total hormone levels often misrepresent the true biological exposure due to variations in binding protein concentrations.
Mechanism
Circulating androgens exist primarily bound to SHBG, with a smaller portion bound to albumin, and a very small fraction remaining unbound or “free.” Only the free and albumin-bound portions can readily diffuse into tissues and exert their biological effects. Clinical calculations, often involving total testosterone and SHBG measurements, are used to derive these bioavailability metrics, providing a more precise gauge of tissue-level androgen exposure and its correlation with clinical symptoms. This calculation is essential because SHBG concentration can be significantly influenced by factors like thyroid status, insulin levels, and age.
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