Not often carried out at no cost, "free" here means the androgen (testosterone) in blood that is not bound to carrier proteins such as sex hormone binding globulin, so it's immediately available for action in the tissues. A more sensitive test than serum testosterone and likely to be increased in the polycystic ovary syndrome. An indirect measure of serum free testosterone, which can be performed instead.

Measurement of testosterone, the chief male sex hormone (androgen) circulating in the blood serum. If increased in women with oligomenorrhea or amenorrhea, indicative of the polycystic ovary syndrome. The free androgen index and free testosterone are more sensitive tests.

The main carrier protein in the blood for the hormone testosterone; measured in conjunction with the serum testosterone to derive the free androgen index.