The free androgen index is a measure of the biologically active testosterone in the blood. It is a ratio of the total testosterone to the level of sex hormone-binding globulin (SHBG). About half of all testosterone in the blood is tightly bound to SHBG and the rest is either free or loosely bound to albumin. Only the portion that is free or bound to albumin is considered biologically active. The portion bound to SHBG is considered biologically inert. Thus, by calculating the ratio of total testosterone to SHBG, one can get a measurement of physiologically active testosterone in the blood. The free androgen index is expressed in the same units and multiplied by 100 to provide a number similar to the free testosterone concentration in blood. Normal free androgen index results are slightly higher than free testosterone levels, however. As a ratio, there are no units. The free androgen index may be a more accurate assessment of free testosterone in women than in men.
More about the relationship between Free Androgen Index and SHBG:
Sex hormone-binding globulin (SHBG) and the free androgen index (FAI) are closely related measures that provide insight into androgen status and metabolic health. Here is a summary of their relationship:
SHBG is a protein that binds to sex hormones like testosterone, regulating their bioavailability. Lower SHBG levels lead to higher levels of unbound or "free" testosterone. The FAI is calculated as (total testosterone / SHBG) x 100 and represents the percentage of total testosterone that is unbound and biologically active.[1]
An elevated FAI, resulting from low SHBG and/or high testosterone, is associated with adverse cardiometabolic risk factors such as insulin resistance, dyslipidemia, and systemic inflammation in both men and women.[2][4][5] In women, a high FAI is a key feature of polycystic ovary syndrome (PCOS) and is linked to reproductive issues like menstrual irregularities and hirsutism.[4]
Conversely, higher SHBG levels lead to lower FAI and are associated with a more favorable cardiometabolic profile.[2][5] Factors like obesity, insulin resistance, and certain medications can suppress SHBG production, raising FAI.[1][3] Age, genetics, and ethnicity also influence SHBG and FAI levels.[3][5]
In summary, SHBG and FAI are inversely related measures that reflect androgen bioavailability. Their levels are influenced by metabolic factors and have implications for reproductive and cardiometabolic health in both sexes.[1][2][3][4][5]
Normal Ranges for Free Androgen Index:
• Male:
− 20 to 29 years: 30.0−128.0
− 30 to 39 years: 24.0−122.0
− 40 to 49 years: 14.0−126.0
− Older than 49 years: 18.0−82.0
• Female:
− 20 to 49 years: 0.4−8.4
− Older than 49 years: 0.4−6.6
Sources:
https://www.labcorp.com/test-menu/25691/free-androgen-index-fai
https://www.labcorp.com/test-menu/35496/testosterone-free-direct
https://www.uptodate.com/contents/overview-of-androgen-deficiency-and-therapy-in-women
Citations:
[1] https://www.ahajournals.org/doi/10.1161/01.cir.0000157697.54255.ce
[2] https://www.sciencedirect.com/topics/biochemistry-genetics-and-molecular-biology/free-androgen-index
[3] https://www.endocrinologiapediatrica.org/modules.php?idarticulo=364&idlangart=EN&name=articulos
[4] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8279613/
[5] https://pubmed.ncbi.nlm.nih.gov/15769764/
Free Androgen Index
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