Free Androgen Index ∞ Area

Free Androgen Index

Meaning

The Free Androgen Index (FAI) is a calculated ratio estimating biologically active testosterone in the bloodstream, representing the portion not bound to sex hormone-binding globulin (SHBG). This index serves as a valuable clinical tool for assessing androgen status when total testosterone levels alone do not accurately reflect physiologically available hormone. It provides a nuanced understanding of an individual’s androgenic environment.

Context

Within the endocrine system, the FAI indicates androgen bioavailability. Testosterone circulates largely bound to proteins like SHBG. SHBG binds testosterone with high affinity. The FAI focuses on the proportion of testosterone not tightly bound to SHBG, representing the fraction readily available to target tissues and receptors for physiological action.

Significance

The FAI’s clinical importance is substantial for evaluating androgen excess or deficiency. For women, elevated FAI can support polycystic ovary syndrome (PCOS) diagnosis and explain symptoms like hirsutism. In men, low FAI may indicate hypogonadism, even with borderline total testosterone, correlating with fatigue or reduced libido. This metric aids precise diagnostic decisions.

Mechanism

The Free Androgen Index is derived mathematically: the ratio of total testosterone to sex hormone-binding globulin (SHBG), often multiplied by a constant. This calculation reflects that only unbound testosterone readily interacts with androgen receptors to elicit biological responses. Accounting for SHBG, which tightly sequesters testosterone, the FAI offers a more accurate representation of the hormone’s active concentration.

Application

Clinically, the FAI is routinely applied in diagnosing and managing hormonal disorders. It investigates causes of infertility or androgenic skin conditions in women. In men, FAI assessment assists in diagnosing hypogonadism when symptoms suggest deficiency despite total testosterone levels not being overtly low. It also monitors hormone replacement therapy effectiveness.

Metric

Assessing the Free Androgen Index necessitates specific serum blood tests, typically a morning blood draw. Laboratory analysis quantifies both total testosterone and sex hormone-binding globulin (SHBG) concentrations. Once these biomarker values are obtained, the FAI is calculated using the formula: (Total Testosterone / SHBG) x 100 (or 1000). The result is interpreted against established reference ranges.

Risk

Improper application or misinterpretation of the Free Androgen Index carries potential clinical risks. Relying solely on FAI without considering the full clinical picture, including patient symptoms and medical history, can lead to inaccurate diagnoses or inappropriate treatment. Conditions altering SHBG levels, like liver disease or certain medications, can artificially inflate or depress the FAI, potentially misleading clinicians.

A focused individual wearing glasses, precise hand gestures signifying meticulous diagnostic assessment during a patient consultation for hormone optimization. This embodies personalized medicine, addressing metabolic health, cellular function and therapeutic efficacy via wellness protocols.

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∞ Subcutaneous Injection
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HRTioJuly 19, 2025