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Reference Notes and Guidelines

Testosterone is present in the circulation both in protein bound (96%) and in non-protein bound ('free') formats. In males, about 60-70 % is bound to Sex Hormone Binding Globulin (SHBG), 30-40% to albumin and less than 3.5% is 'free'. Albumin-bound and free testosterone, together, are referred to as the biologically active or bioavailable testosterone. The Free Androgen Index (FAI) measures androgen status, an alternate to measuring free testosterone, and is calculated by the ratio of Total Testosterone to SHGB as a percentage. In women, a raised FAI may be helpful in the diagnosis of hirsutism or polycystic ovary syndrome. The correlation of FAI in adult males with disease or testosterone deficiency has not been validated. Bioavailable Testosterone is estimated by using the total Testosterone, SHBG and Albumin in a complex formula. A "Free & Bioavailable Testosterone Calculator" (Ref 2) may be used to estimate free Testosterone. Please note that calculated free and bioavailable testosterone levels are reliable in most clinical situations, but should not be relied upon in situations with potential massive interference by steroids binding to SHBG; e.g. in women during pregnancy, in men during treatment inducing high levels of DHT (e.g. transdermal DHT, oral testosterone) or mesterolon.

References:

1. Siemens Healthcare Diagnostic Products Ltd, Technical Bulletins 2010 06-10, and ZB158-8, 1997 for Immulite 2000 XPi.

2. Hormonology department, University Hospital of Ghent, Belgium. Free & Bioavailable Testosterone Calculator. (http://www.issam.ch/freetesto.htm)