Free testosterone is the small amount of testosterone in the bloodstream that is not attached to a protein. Because it is unbound, free testosterone is more available to interact with tissues than protein-bound testosterone. Women produce smaller amounts of testosterone through the ovaries, adrenal glands and conversion from other hormones.
Published in 2025, a Society for Endocrinology guideline advised caution when using the free androgen index if SHBG is below 30 nmol/L, because the calculation may overestimate free androgen levels.
Free Testosterone in Women at a Glance
| Term | Meaning | Why it matters |
|---|---|---|
| Total testosterone | Free testosterone plus protein-bound testosterone | The most commonly measured testosterone result |
| Free testosterone | Testosterone that is not attached to a binding protein | Estimates the unbound fraction available to tissues |
| SHBG | Sex hormone-binding globulin, a protein that binds testosterone | Higher SHBG usually means less free testosterone; lower SHBG can mean more |
| Bioavailable testosterone | Free testosterone plus testosterone loosely bound to albumin | Another estimate of testosterone available to tissues |
| Free androgen index | A calculation based mainly on total testosterone and SHBG | An estimate, not a direct free testosterone measurement |
Most testosterone in the blood is bound to SHBG or albumin. Changes in SHBG can therefore alter free testosterone even when total testosterone remains within the laboratory reference range.
Why Do Doctors Measure Free Testosterone in Women?
Doctors usually measure free testosterone when a woman has signs of androgen excess, also called hyperandrogenism. These signs can include:
- Excess facial or body hair, known as hirsutism
- Persistent or severe acne
- Scalp hair thinning or female pattern hair loss
- Irregular, infrequent or absent menstrual periods
- Infertility
- Rapidly increasing muscle mass or other masculinizing changes
Polycystic ovary syndrome, or PCOS, is a common cause of high androgen levels in women. Other possible causes include certain medications, nonclassic congenital adrenal hyperplasia, severe insulin resistance and, rarely, an androgen-producing ovarian or adrenal tumor.
Clinicians interpret free testosterone alongside symptoms, menstrual history, total testosterone and SHBG. They may also order tests such as DHEAS, androstenedione and 17-hydroxyprogesterone.
What Is the Difference Between Free and Total Testosterone?
Total testosterone measures all testosterone in the blood. It includes:
- Testosterone tightly bound to SHBG
- Testosterone loosely bound to albumin
- Unbound, or free, testosterone
Free testosterone makes up only a small fraction of total testosterone. That fraction can change when SHBG changes. For example, a woman may have a total testosterone result within the laboratory range but a higher free testosterone level if her SHBG is unusually low.
This is why a clinician may assess free testosterone when symptoms suggest androgen excess but total testosterone does not appear high.
What Does a High Free Testosterone Level Mean?
A high free testosterone level may support a diagnosis of biochemical hyperandrogenism, especially when hirsutism, acne or irregular periods are also present. High free testosterone often occurs with PCOS, but the result alone cannot show that PCOS is the cause. Clinicians may need to exclude other explanations.
Seek prompt medical assessment if androgen-related changes appear quickly, including:
- Rapid facial or body hair growth
- A noticeably deeper voice
- Clitoral enlargement
- Sudden scalp hair loss
- Rapidly worsening acne or increased muscularity
Rapid progression or virilization can point to a more serious source of androgen excess and needs medical evaluation.
What Does a Low Free Testosterone Level Mean?
A low free testosterone result alone cannot establish testosterone deficiency in a woman. Medical societies have not established a reliable female testosterone-deficiency syndrome, a universal symptom threshold or a single free testosterone value that explains fatigue, low mood, low libido or other nonspecific symptoms.
A low result may reflect:
- High SHBG
- Estrogen-containing medication
- Certain endocrine or metabolic conditions
- Ovarian or adrenal disorders
- Medication effects
- Normal biological variation
- Differences between laboratory testing methods
The Endocrine Society recommends against routinely treating women solely because a blood test shows low androgen levels. Treatment decisions should follow a broader clinical assessment rather than the free testosterone number alone.
How Is Free Testosterone Tested?
Clinicians usually assess free testosterone with a blood test. Testing methods vary in accuracy because testosterone levels in women are relatively low. Direct free testosterone assays can be unreliable, so clinicians may use:
- Free testosterone measured by equilibrium dialysis
- A calculated free testosterone result based on total testosterone, SHBG and albumin
- A reliable total testosterone assay interpreted alongside SHBG and symptoms
Women do not have one universal free testosterone reference range. Results vary according to the assay, laboratory, age, menstrual status and whether a woman is premenopausal or postmenopausal. Compare the result with the reference range supplied by the laboratory that performed the test, rather than with a range found online.
The free androgen index, or FAI, is one way to estimate free testosterone. It is a calculation rather than a direct measurement, and its accuracy can change when SHBG is low.
How Should You Interpret a Free Testosterone Result?
Free testosterone is useful for assessing testosterone availability, but it is not a diagnosis on its own. A clinician will usually consider:
- Total testosterone and SHBG
- Albumin, when relevant to the calculation
- The menstrual pattern
- Acne, hair growth or hair loss
- Current medications and hormonal contraception
- Age and menopausal status
- The laboratory's testing method and reference range
A high free testosterone result with androgen-related symptoms deserves evaluation. A low result alone does not establish testosterone deficiency or justify testosterone treatment.