01-29-2006, 08:40 AM
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Join Date: Jul 2005
The answer to your question is in the attached study. (now how they did the research is a bit too complex for me to post in a few sentences). http://jcem.endojournals.org/cgi/con...ull/84/10/3666
Some interesting sentences:
The apparent free testosterone
(T) concentration obtained by equilibrium dialysis (AFTC)
as well as the fraction of serum T not precipitated by 50% ammonium sulfate concentration
(non-SHBG-T; SHBG, sex hormone-binding globulin), often referred to as bioavailable T, appear to represent reliable indexes of biologically readily available T, but are not well suited for clinical routine, being too time consuming
Several other parameters
have been used without complete validation
direct immunoassay of free T with a labeled T analog (aFT),
calculation of free T (FT) from total T
and immunoassayed SHBG concentrations (iSHBG),
and the free androgen index (FAI = the ratio 100T/iSHBG).
In the view of substantial discrepancies in the literature concerning the free or bioavailable T levels, we compared AFTC, FT, aFT, FAI, and non-SHBG-T levels in a large number of sera with SHBG capacities varying from low, as in hirsute women, to extremely high as in hyperthyroidism. All these indexes of bioavailable T correlated significantly with the AFTC concentration; AFTC and FT values were almost identical under all conditions studied, except during pregnancy. Values for aFT, however, were only a fraction of either AFTC or FT, the fraction varying as a function of SHBG levels. Also, the FAI/AFTC ratio varied as a function of the SHBG levels, and hence, neither aFT nor FAI is a reliable index of bioavailable T
Their conclusion: The FT value, obtained by calculation from T and SHBG as determined by immunoassay, appears to be a rapid, simple, and reliable index of bioavailable T, comparable to AFTC and suitable for clinical routine, except in pregnancy.
I hope this will help in answering your question? I realise I did not give a directg answer to your question, but I hope the link will help you out!