The literature states that the FAI is suitable for females but not really for males, where alternatively, the Vermeulen calculation is commonly used to calculate free testosterone.
The Free androgen index correlates well with free testosterone in women but not men. This is due to the fact that testosterone production is regulated by gonadotropin feedback in men and changes in Sex hormone binding globulin (SHBG), which alter free testosterone concentrations, will be compensated by autoregulation of Testosterone production, but not so in women.
A large portion of the circulating testosterone in women is derived from the peripheral conversion of adrenal dehydroepiandrosterone and dehydroepiandrosterone sulfate that also is not subject to feedback control. Because SHBG is present in such large excess in women (10–100:1), free testosterone concentrations are driven primarily by SHBG abundance. In addition, Testosterone excess in women lowers SHBG concentrations, which raises the FT concentration and contributes to the strong correlation of 1/SHBG with FT.
See Ann Clin Biochem. 2006 Sep;43(Pt 5):389-97.
Calculated free testosterone in men: comparison of four equations and with free androgen index.
Ho CK, Stoddart M, Walton M, Anderson RA, Beckett GJ.
The concentration of total testosterone is in the order of 1 to 3 nmol/L in women and 10 to 30 nmol/L in men. The lower limits of SHBG concentrations in men and women is around 10 - 20 nmol/L ie more than enough to bind testosterone in any woman, but not enough to bind all the testosterone in many men. In men, albumin also becomes an important determinant of how all that testosterone is carried in the blood and how much is free. In summary, the FAI, which is a simple ratio between SHBG and testosterone is too simple to explain men's free testosterone levels.
Sartorius G, Ly LP, Sikaris KA, McLachlan R, Handelsman DJ, “Predictive accuracy and sources of variability in calculated free testosterone estimates”, Ann Clin Biochem 2009;46:137-143.
Basically as explained earlier it is the prezence of sexhormone binding globulins in women which affects the total and free testosterone levels while in men SHBG levels are not very high as they are estrogen driven and Kumar et al explain the vermuelen and other formulas used to derive not only free testosterone but bioavailable testosterone
http://www.ncbi.nlm.nih.gov/pubmed/15296480
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