Free Testosterone and SHBG Interpretation in Women
Free testosterone is more sensitive than total testosterone for detecting androgen excess in women, and should be calculated from total testosterone and SHBG measurements rather than measured by direct immunoassay. 1
Key Physiological Principles
SHBG is the critical determinant of free testosterone levels. SHBG is a high-affinity, low-capacity binding protein that determines what fraction of plasma testosterone is free or bound to other proteins (primarily albumin). The bioactive portion of testosterone includes free testosterone plus a portion of albumin-bound testosterone 1.
Factors Affecting SHBG Levels:
- Increased by: Estrogen, hyperthyroidism, liver disease
- Decreased by: Androgens, hyperinsulinemia/insulin resistance, hypothyroidism, obesity, metabolic syndrome 1, 2, 1
Clinical Interpretation
Why free testosterone matters more: Women with hyperandrogenism commonly have relatively low SHBG levels, which means their total testosterone may appear normal while free testosterone is elevated. This makes free testosterone more sensitive for detecting excess androgen production 1.
Timing Considerations:
- Measure during early morning, midfollicular phase of menstrual cycle
- Testosterone varies ~25% around the mean during the cycle
- Levels are highest in early morning and slightly elevated at midcycle 1
Laboratory Measurement Approach
The most reliable method is calculated free testosterone, NOT direct immunoassay. 1, 3
Recommended Testing Algorithm:
- Initial screening: Measure total testosterone using validated assay (preferably LC-MS/MS)
- If clinical suspicion persists with normal total testosterone: Measure both total testosterone AND SHBG
- Calculate free testosterone using the Vermeulen equation or similar validated formula based on:
- Total testosterone concentration
- SHBG concentration
- Albumin concentration
- Known dissociation constants 1
Critical Pitfall to Avoid:
Never use direct immunoassay for free testosterone - these methods are "extraordinarily inaccurate" and "seriously flawed" despite being simple and inexpensive 3. The values generated are unreliable for clinical decision-making.
Assay Quality Issues
Standard hospital automated assays are often inadequate for measuring testosterone in women. 1 The challenges include:
- Systematic differences between assays
- Excessively broad normal ranges
- Poor accuracy at low female testosterone levels
- No uniform laboratory standards for free testosterone 1
Use specialty laboratories with validated assays or LC-MS/MS methods when available for more accurate androgen determinations 1, 2.
Alternative Indices
Free Androgen Index (FAI) can be used as a practical alternative:
- Calculated as: (Total testosterone / SHBG) × 100
- Shows reasonable correlation with calculated free testosterone
- More widely available than specialized free testosterone measurements 4, 5
Clinical utility: Studies show FAI and calculated free testosterone detect hyperandrogenism in 89-95% of PCOS patients versus only 36% detection with total testosterone alone 5.
When to Measure Additional Androgens
Routine measurement of other androgens (DHEAS, androstenedione) is of little utility in most populations 1. However, consider DHEAS when:
- Total testosterone is twice the upper limit of normal (suggests adrenal source)
- DHEAS >700 μg/dL suggests androgen-secreting tumor requiring imaging 2
Special Populations
For high-risk ethnic groups (Ashkenazi Jews, Hispanics, Slavics), also measure early-morning follicular-phase 17-hydroxyprogesterone to screen for nonclassic congenital adrenal hyperplasia, as free testosterone variability may miss occasional cases 1.