Free androgen index

Free Androgen Index or FAI is a ratio used to determine abnormal androgen status in humans. The ratio is the total testosterone level divided by the sex hormone binding globulin (SHBG) level, and then multiplying by a constant, usually 100. The concentrations of testosterone and SHBG are normally measured in nanomols per liter. FAI has no units.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.The Free Androgen Index (FAI) was initially proposed as a measure for assessing the circulating testosterone availability in female hirsutism. The extension of its use, by a number of investigators, to males has not been formally justified. Free Androgen Index or FAI is a ratio used to determine abnormal androgen status in humans. The ratio is the total testosterone level divided by the sex hormone binding globulin (SHBG) level, and then multiplying by a constant, usually 100. The concentrations of testosterone and SHBG are normally measured in nanomols per liter. FAI has no units. F A I = 100 × ( T o t a l   T e s t o s t e r o n e S H B G ) {displaystyle FAI=100 imes ({frac {Total Testosterone}{SHBG}})} The majority of testosterone in the blood does not exist as the free molecule. Instead around half is tightly bound to sex hormone binding globulin, and the other half is weakly bound to albumin. Only a small percentage is unbound, under 3% in males, and less than 0.7% in females. Since only the free testosterone is able to bind to tissue receptors to exert its effects, it is believed that free testosterone is the best marker of a person's androgen status. However, free testosterone is difficult and expensive to measure, and many laboratories do not offer this service.