Normal Free Testosterone Levels in Women
The normal range for free testosterone in women is approximately 0.3-1.9 pg/mL (0.3-0.65 pg/mL in postmenopausal women), with levels typically being 15-25 times lower than in men.
Reference Ranges for Free Testosterone in Women
Free testosterone is considered the metabolically active fraction of testosterone and provides more clinically relevant information than total testosterone measurements in many cases. The reference ranges vary based on several factors:
By Age and Reproductive Status:
- Premenopausal women with regular menstrual cycles: 0.3-1.9 pg/mL (3-65 pmol/L) 1
- Postmenopausal women: 0.3-0.65 pg/mL (approximately 0.52 ± 0.12 ng/dL) 2
- Pregnant women: Significantly lower than non-pregnant women, approximately 0.33 ± 0.23 ng/dL 2
By Menstrual Phase:
- Follicular phase: 0.61 ± 0.20 ng/dL
- Luteal phase: 0.56 ± 0.35 ng/dL 2
No significant differences exist between follicular and luteal phases of the menstrual cycle.
Measurement Methods and Considerations
The accuracy of free testosterone measurements depends significantly on the methodology used:
- Equilibrium dialysis is considered the gold standard method 3
- Calculated free testosterone using the law of mass action (requiring total testosterone and SHBG measurements) correlates well with equilibrium dialysis (r = 0.99) 3
- Direct RIA (analog method) shows poor correlation with equilibrium dialysis and is not recommended due to high systematic bias and variability 3
- Free androgen index (100 × T/SHBG) correlates well with equilibrium dialysis (r = 0.93) but provides a unitless number 3
Factors Affecting Free Testosterone Levels
Several factors can influence free testosterone levels in women:
- Obesity: Can decrease SHBG levels, potentially increasing free testosterone 4
- Oral contraceptives: Increase SHBG levels, decreasing free testosterone 4
- Liver disease: Can affect SHBG production and alter free testosterone levels 4
- Polycystic ovary syndrome (PCOS): Often associated with elevated free testosterone levels 5
- Thyroid dysfunction: Can affect SHBG levels and alter free testosterone 4
Clinical Implications
Free testosterone levels outside the normal range may indicate:
- Low levels (<0.3 pg/mL): Associated with decreased libido, dyspareunia, osteoporosis, depression, and increased total body fat mass 6
- High levels (>1.9 pg/mL): May indicate hyperandrogenic states such as PCOS, hirsutism, or adrenal disorders 5
Testing Recommendations
When evaluating for possible testosterone abnormalities:
- Obtain morning samples when possible
- Consider measuring both total testosterone and SHBG to calculate free testosterone
- Use equilibrium dialysis or calculated free testosterone rather than direct assays
- Interpret results in context of clinical presentation and other hormone levels
- Consider repeat testing if values are borderline or inconsistent with clinical picture
Common Pitfalls in Free Testosterone Testing
- Using direct/analog assays that have poor correlation with reference methods
- Failure to consider SHBG levels when interpreting total testosterone
- Not accounting for oral contraceptive use or other medications that affect SHBG
- Relying on a single measurement rather than confirming abnormal values
- Using laboratories with inadequate quality control for the low concentrations found in women
Free testosterone measurement is particularly important in evaluating women for androgen deficiency or excess, as it represents the biologically active fraction of testosterone that is available to tissues.