Free Androgen Index (FAI) is a calculated ratio that estimates how much testosterone is biologically active (available to tissues) in your body. It’s not a hormone measured directly. Instead, FAI compares total testosterone with sex hormone-binding globulin (SHBG), the main protein that binds and inactivates testosterone in the bloodstream. A higher FAI generally indicates more bioavailable testosterone relative to binding capacity.

FAI is calculated using your total testosterone result and your SHBG result to estimate the “free” or active fraction of testosterone. Because SHBG binds most circulating testosterone, the ratio helps show whether symptoms may reflect true androgen excess/deficiency or changes in binding proteins. For best interpretation, FAI should be reviewed alongside total testosterone, SHBG, and your clinical symptoms.

SHBG acts like a carrier and buffer for testosterone, binding a large portion of circulating hormone. Bound testosterone is largely inactive, while unbound (“free”) testosterone can enter cells and drive effects on energy, muscle, libido, mood, metabolism, and reproductive function. When SHBG rises, FAI tends to fall; when SHBG drops, FAI often rises - even if total testosterone doesn’t change much.

FAI testing helps reveal how much active testosterone is available, which can clarify symptoms linked to androgen imbalance. It can help assess irregular periods, excess facial/body hair (hirsutism), acne, scalp hair thinning, and unexplained weight gain. FAI is also useful for flagging androgen excess patterns commonly associated with polycystic ovary syndrome (PCOS) and for tracking response to therapy over time.

A high FAI in women often suggests increased free testosterone availability, commonly seen with PCOS. Elevated androgen activity may contribute to irregular cycles, acne, unwanted hair growth, scalp thinning, and insulin resistance. High FAI can result from higher total testosterone, lower SHBG, or both. Because obesity and insulin resistance can lower SHBG, FAI can rise even without a large increase in testosterone production.

Low FAI typically indicates reduced bioavailable testosterone relative to binding proteins, often due to high SHBG or low testosterone production. In men, this pattern may align with fatigue, reduced libido, difficulty building muscle, reduced muscle tone, mood changes, and slower recovery. Importantly, total testosterone can appear “normal” while FAI is low, which is why pairing FAI with symptoms and SHBG matters.

In women, low FAI is less commonly flagged but may occur with very high SHBG states or ovarian insufficiency. Very low androgen availability may contribute to low energy, diminished vitality, and - over time - effects on bone density and well-being, particularly after menopause. Because women naturally have a smaller androgen pool, “optimal” results are often in the low-normal range rather than mid-to-high values.

FAI can shift due to factors that alter SHBG. Obesity, insulin resistance, hypothyroidism, and liver disease tend to lower SHBG and raise FAI independent of true androgen excess. Pregnancy, oral estrogen, and hyperthyroidism often raise SHBG and lower FAI. Because FAI is a calculated ratio, changes in SHBG can meaningfully change results even when total testosterone changes only slightly.

FAI is most useful when interpreted with total testosterone, SHBG, and your clinical picture. This helps distinguish whether symptoms are driven by true androgen excess/deficiency or by altered binding proteins. For example, someone may have normal total testosterone but low FAI due to high SHBG, or a high FAI driven mainly by low SHBG from metabolic factors. Symptom context improves accuracy and clinical relevance.

FAI is not a direct measurement of free testosterone; it’s an estimate derived from total testosterone and SHBG. A common misconception is that FAI alone “diagnoses” conditions like PCOS - FAI can support assessment but should be paired with symptoms and related labs. Another misconception is that high FAI always means high testosterone production; low SHBG from metabolic issues can raise FAI even without major testosterone increases.