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 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 females, and less than 0.7% in males. 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.
The free androgen index is intended to give a guide to the free testosterone level, but it is not very accurate. Consequently, there are no universally agreed 'normal ranges', and levels slightly above or below quoted laboratory reference ranges may not be clinically significant.
Reference ranges depend on the constant in the calculation - 100 is used in the formula above, and the following suggested ranges are based on this. As with any laboratory measurement, however, it is vital that results are compared against the reference range quoted for that laboratory. Neither FAI nor free or total testosterone measurements should be interpreted in isolation; as a bare minimum, gonadotropin levels should also be measured.
As a guide, in healthy adult men typical FAI values are 30-150. Values below 30 may indicate testosterone deficiency, which may contribute to fatigue, erectile dysfunction, tall stature, osteoporosis and loss of secondary sex characteristics. In women, androgens are most often measured when there is concern that they may be raised (as in hirsutism or the polycystic ovary syndrome). Typical values for the FAI in women are 7-10.
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