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.
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
Adult men with too little testosterone may lose lean muscle mass, become fatigued, feel weak, and develop depression. Low testosterone can reduce libido and contribute to erectile dysfunction. Gynecomastia—the development of breast tissue in men—is another symptom of low testosterone. Boys with low testosterone may fail to develop secondary sex characteristics such as deepened voice, pubic hair, and facial hair. Low testosterone is less problematic for women, but it can interfere with sexual function.
Some specific causes of low free androgen index are:
- Hypogonadism
- Testicular injury
- Testicular cancer
- Testicular cancer treatment
- Type 2 diabetes mellitus
- Obesity
- Chronic liver disease
- Chronic kidney disease
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A high free androgen index value indicates that there is too much biologically active testosterone in the blood. Men with too much testosterone may be overly aggressive, develop cystic acne, and have increased body hair. In general, however, adult men may not notice symptoms of too much testosterone. Women with high testosterone develop many noticeable symptoms and signs such as unusual hair growth, obesity, hoarseness, and menstrual irregularities. Children with high testosterone may develop precocious puberty.
Some specific causes of high free androgen index are:
- Polycystic ovarian syndrome
- Testosterone-secreting tumor
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