The Free Androgen Index (FAI) is a measure of the amount of biologically active, or free, testosterone in a man's bloodstream relative to the total amount of testosterone in the blood. It is calculated by multiplying the total testosterone level by 100 and dividing it by the level of a protein called sex hormone-binding globulin (SHBG).

 In men, testosterone is an important hormone that plays a vital role in the development of male characteristics, such as muscle mass, bone density, and body hair. It also helps regulate sex drive, fertility, and mood.

The FAI is used to assess whether a man's testosterone levels are within the normal range. If the FAI is low, it may indicate that the man has low levels of free testosterone, which can lead to symptoms such as low sex drive, erectile dysfunction, and decreased muscle mass. On the other hand, a high FAI may indicate that a man has high levels of testosterone, which can lead to symptoms such as increased body hair, acne, and aggression.

The FAI is a great tool to see how much free testosterone you have in your body. However, it is just one measure of testosterone levels, and other factors such as age, health status, and medications can affect testosterone levels and the interpretation of FAI results. Therefore, it is important to consult with a healthcare provider to determine the best course of action based on a comprehensive evaluation of a patient's individual situation.