Free Androgen Index Unit Converter

The Free Androgen Index (FAI) is a calculated ratio that estimates how much testosterone is biologically available to act on tissues. It is derived by dividing total testosterone by sex hormone-binding globulin (SHBG) and multiplying by 100.

Most testosterone in the blood is bound to proteins, primarily SHBG (about 44%) and albumin (about 50%). Only a small percentage (under 3% in men, less than 0.7% in women) circulates unbound as free testosterone. Since only free testosterone can enter cells and activate androgen receptors, FAI provides a practical estimate of androgen availability.

FAI is particularly useful for detecting androgen excess in women, where it correlates well with free testosterone. In men, its accuracy is more limited due to hormonal feedback mechanisms that compensate for changes in SHBG.

FAI is a calculated value, not a directly measured hormone. It is derived from two blood measurements: total testosterone (produced mainly in the testes in men, ovaries and adrenal glands in women) and SHBG (a transport protein produced by the liver).