The FAI is often used as a surrogate for FT, and the FAI correlates well with FT in women but not men . Because T production is regulated by gonadotropin feedback in men, changes in SHBG, which alter FT concentrations, will be compensated by autoregulation of T production but not so in women. In addition, much circulating T 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), FT concentrations are driven primarily by SHBG abundance. In addition, T excess in women lowers SHBG concentrations, which raises the FT concentration and contributes to the strong correlation of 1/SHBG with FT. 
Testosterone is significantly correlated with aggression and competitive behaviour and is directly facilitated by the latter. There are two theories on the role of testosterone in aggression and competition.  The first one is the challenge hypothesis which states that testosterone would increase during puberty thus facilitating reproductive and competitive behaviour which would include aggression.  Thus it is the challenge of competition among males of the species that facilitates aggression and violence.  Studies conducted have found direct correlation between testosterone and dominance especially among the most violent criminals in prison who had the highest testosterone levels.  The same research also found fathers (those outside competitive environments) had the lowest testosterone levels compared to other males. 
Intramuscular testosterone preparations have been the mainstay of testosterone replacement therapy since the 1950s, and they are one of the most popular forms of testosterone for TRT. Only recently has the usage of testosterone gels (like Androgel and Axiron) surpassed injectable testosterone usage for TRT. As of 2014, approximately 60% of TRT users use testosterone gels, while 35% use injectable testosterone preparations (According to Endo Pharmaceuticals FDA filing for Aveed). With this being said, the surge in testosterone gel usage may largely be attributed to the advertising by the pharmaceutical companies promoting these gels.