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Free Androgen Index (FAI)

The free androgen index is used to estimate levels of physiologically active testosterone.

In most men and women, more than 50% of the total circulating testosterone is bound to the sex hormone-binding globulin (SHBG) and most of the remainder is bound to albumin. Testosterone that binds to SHBG is not readily available because of SHBG's high affinity for testosterone. Thus, bound testosterone with SHBG is considered to be biologically inactive. Albumin, on the other hand, has a much lower binding affinity for testosterone but it binds a significant portion of total testosterone because albumin is at much higher plasma concentrations than SHBG. The rapid release of "weakly bound" testosterone from albumin results in the availability of substantially all of the albumin-bound testosterone for interaction with cellular receptors for steroids.

The sum of free testosterone and albumin-bound testosterone is referred to as bioavailable testosterone. The concentration of testosterone in its free and various bound forms depends essentially on the total concentration of testosterone and the relative concentrations of SHBG and albumin. Increasing SHBG will decrease the concentration of both free and bioavailable testosterone for a given total testosterone concentration.

The free androgen index can be used to assess physiologically active testosterone. This index is calculated as the ratio of total testosterone divided by the concentration of SHBG.

The free androgen index is considered to be a better indicator for monitoring androgen levels than free testosterone. 



Important Note

Laboratory test results are the most important parameter for the diagnosis and monitoring of all pathological conditions. 70%-80% of diagnostic decisions are based on laboratory tests. The correct interpretation of laboratory results allows a doctor to distinguish "healthy" from "diseased".

Laboratory test results should not be interpreted from the numerical result of a single analysis. Test results should be interpreted in relation to each individual case and family history, clinical findings, and the results of other laboratory tests and information. Your personal physician should explain the importance of your test results.

At Diagnostiki Athinon we answer any questions you may have about the test you perform in our laboratory and we contact your doctor to get the best possible medical care.

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