Normal Free Testosterone Levels
Normal free testosterone levels in adult men range from approximately 66-309 pg/mL (229-1072 pmol/L) across all ages, with higher levels of 120-368 pg/mL (415-1274 pmol/L) expected in younger men aged 19-39 years, measured by standardized equilibrium dialysis. 1
Age-Specific Reference Ranges
Free testosterone levels decline with age and should be interpreted using age-appropriate thresholds:
- Men 19-39 years old: The normative range is 120-368 pg/mL, with the 2.5th percentile at 120 pg/mL and 97.5th percentile at 368 pg/mL 1
- All adult men (19+ years): The broader normative range is 66-309 pg/mL, with the 2.5th percentile at 66 pg/mL and 97.5th percentile at 309 pg/mL 1
- Men in their 40s: Age-specific lower threshold is approximately 409 ng/dL for total testosterone, which correlates with appropriate free testosterone levels 2
Critical Measurement Considerations
Free testosterone must be measured by equilibrium dialysis in specific clinical scenarios, particularly when total testosterone results are ambiguous:
- Men with obesity often have low total testosterone due to decreased sex hormone-binding globulin but may have normal free testosterone levels 3
- When total testosterone is near the lower limit of normal (280-400 ng/dL), free testosterone measurement is essential because total testosterone lacks sufficient sensitivity in this range 4
- Morning fasting samples provide the most accurate assessment, as testosterone levels fluctuate throughout the day with peak levels occurring in the morning 5, 6
Relationship to Total Testosterone
Understanding the correlation between total and free testosterone helps guide testing:
- Total testosterone >350-400 ng/dL reliably predicts normal free testosterone with 96.8-98.2% sensitivity 4
- Total testosterone <280 ng/dL has 91% sensitivity for detecting low free testosterone but only 73.7% specificity 4
- Total testosterone 280-350 ng/dL represents a gray zone where free testosterone measurement is mandatory, as total testosterone alone is not sensitive enough to exclude hypogonadism 4
Clinical Context for Interpretation
Free testosterone levels are influenced by multiple factors beyond age:
- Body mass index negatively correlates with free testosterone levels - men with BMI >29 kg/m² have significantly lower levels 7
- Chronic diseases (diabetes, heart disease, hypertension, cancer) decrease free testosterone concentrations 7
- Smoking tends to increase testosterone levels, while alcohol consumption >6 drinks/day decreases levels 7
- Sex hormone-binding globulin levels inversely correlate with free testosterone 1
Common Diagnostic Pitfalls
The 300 ng/dL cutoff for total testosterone was derived from older men and is inappropriately low for younger men, who should have higher thresholds:
- Up to 25% of men receiving testosterone therapy do not meet proper diagnostic criteria for testosterone deficiency 5, 8
- Nearly half of men on testosterone therapy never have their levels checked after starting treatment 5, 8
- Free testosterone measurement is essential in obese men, as they may have falsely low total testosterone with normal free testosterone 3
Diagnostic Algorithm
When evaluating suspected testosterone deficiency:
- Obtain two morning (8-10 AM) total testosterone measurements on separate occasions - both must be low before proceeding 3, 5
- If total testosterone is <280 ng/dL, proceed with hypogonadism workup including free testosterone by equilibrium dialysis 4
- If total testosterone is 280-400 ng/dL, free testosterone measurement by equilibrium dialysis is mandatory to confirm deficiency 4
- If total testosterone is >400 ng/dL, hypogonadism is effectively ruled out and free testosterone measurement is unnecessary 4
- In men with obesity, always measure free testosterone by equilibrium dialysis regardless of total testosterone level 3