The Free Androgen Index (FAI) is a calculated marker used by healthcare providers to assess the level of biologically active androgens, such as testosterone, within the body. Androgens play important roles in both men and women, affecting muscle mass, bone density, mood, and reproductive health. Since the effects of these hormones depend on the amount that can freely interact with tissues, the FAI provides a more insightful measure of hormonal activity than simply measuring the total amount of the hormone in the blood. The FAI serves as a standard diagnostic tool for evaluating conditions related to androgen excess or deficiency.
Components and Calculation of the Free Androgen Index
The Free Androgen Index is derived from measuring two specific components in a blood sample: Total Testosterone (T) and Sex Hormone-Binding Globulin (SHBG). Total Testosterone represents all the testosterone circulating in the bloodstream. The majority of this circulating testosterone is tightly bound to SHBG, a protein produced primarily by the liver, making it unavailable for use by the body’s cells.
SHBG acts as a carrier protein, binding to sex hormones like testosterone and rendering them biologically inactive. Only a small fraction of the total testosterone remains unbound or “free,” and this free androgen can easily enter cells to exert its effects. The FAI calculation is designed to estimate the quantity of this biologically available testosterone by factoring in the binding capacity of SHBG.
The standard formula for calculating the Free Androgen Index is: FAI = (Total Testosterone / SHBG) × 100. Concentrations are typically measured in nanomoles per liter (nmol/L), and multiplying the ratio by 100 converts the result into an index value. This calculated ratio offers a practical and cost-effective estimate of biologically active androgen levels, avoiding more complex and expensive direct measurement techniques.
Clinical Significance: Why FAI is Measured
The FAI offers a more accurate picture of a person’s hormonal status compared to Total Testosterone measurement alone, which can be misleading if SHBG concentration is abnormal. For example, a person might have a Total Testosterone level within the standard reference range, yet still experience symptoms of androgen deficiency or excess.
This discrepancy occurs because high SHBG binds more testosterone, leaving less active free form, even if the total level appears normal. Conversely, low SHBG means less testosterone is bound, resulting in a higher amount of biologically active free testosterone. By using the ratio of Total Testosterone to SHBG, the FAI effectively accounts for these variations in the binding protein, providing a reflection of the hormonal activity at the tissue level.
Interpreting Elevated FAI Levels
An elevated Free Androgen Index indicates hyperandrogenism, meaning an excess of biologically available androgen activity. In women, a high FAI is frequently a key diagnostic marker for Polycystic Ovary Syndrome (PCOS), a common endocrine disorder affecting reproductive-aged women. Elevated androgen activity in PCOS can manifest as physical symptoms like hirsutism (excessive hair growth on the face, chest, or back), acne, and male-pattern hair thinning.
Elevated FAI levels are strongly linked to metabolic dysfunction. Insulin resistance and obesity suppress SHBG production, which drives up the FAI and increases active testosterone available to tissues. Women with persistently high FAI face an increased risk of developing metabolic syndrome and Type 2 diabetes.
Other causes of an elevated FAI include certain adrenal gland disorders, such as congenital adrenal hyperplasia or Cushing syndrome, or the use of specific medications. The FAI is a sensitive measure for detecting hyperandrogenemia in women, with values of 5 and above often suggesting a need for further evaluation for PCOS.
Understanding Low FAI Results
A low Free Androgen Index score suggests hypoandrogenism, indicating a deficiency in biologically active testosterone. This can be caused by endocrine or chronic health issues that either reduce total testosterone production or increase SHBG levels. For instance, conditions like hypogonadism (a decline in gonadal function) can result in low androgen levels reflected in a reduced FAI.
In men, a low FAI value (often below 30) may indicate testosterone deficiency contributing to symptoms like fatigue, reduced libido, and decreased muscle mass. Women with low FAI levels may experience persistent fatigue, diminished sexual desire, and irregular menstrual cycles. Elevated SHBG levels, caused by factors like hyperthyroidism, high estrogen states, or the use of certain medications, also contribute to a lower FAI by binding more available testosterone.