Normal Bioavailable Testosterone Levels in Adult Males
The normal range for bioavailable testosterone in adult males is approximately 2.30-2.72 nmol/L, with levels below this range potentially indicating testosterone deficiency. 1
Understanding Testosterone Measurements
- Total testosterone is generally considered normal in the range of 300-800 ng/dL (10.4-27.7 nmol/L) in most laboratories 2, 3
- Bioavailable testosterone includes both free testosterone and albumin-bound testosterone, representing the portion of testosterone that is physiologically active and available to tissues 1, 4
- Morning measurements are critical as testosterone levels naturally fluctuate throughout the day, with highest levels typically occurring in the morning 3
- The diagnosis of low testosterone should only be made after two total testosterone measurements are taken on separate occasions, both conducted in early morning fashion 3
Age-Related Reference Ranges
- Bioavailable testosterone levels decrease with age in healthy men 5, 4
- In men with age-related low testosterone, bioavailable testosterone measurement is more sensitive than total testosterone for detecting age-related decreases 4
- For men in their 40s, 50s, 60s, and 70s, the lower limits (2.5th percentile) of total testosterone are approximately 8.7.5,6.8, and 5.4 nmol/L respectively 5
Clinical Implications
- Testosterone deficiency in males is associated with energy imbalance, impaired glucose control, reduced insulin sensitivity, dyslipidemia, increased abdominal fat mass, and reduction in lean body mass 2
- Low bioavailable testosterone can affect sexual function even when total testosterone appears normal 2
- The American College of Physicians suggests that clinicians discuss testosterone treatment in men with age-related low testosterone with sexual dysfunction who want to improve sexual function 2
- Treatment should be discontinued if there is no improvement in sexual function within 12 months 2
When to Measure Bioavailable Testosterone
- When total testosterone concentrations are found to be around the lower limit of normal (approximately 9.0 nmol/L), bioavailable testosterone should be measured 6
- Bioavailable testosterone is particularly important to measure in men with obesity, as they may have low total testosterone due solely to low sex hormone-binding globulin but normal free testosterone levels 2
- Total testosterone levels of less than 8 nmol/L highly support a diagnosis of hypogonadism, whereas levels greater than 12 nmol/L are likely to be normal 7
- For values between 8 and 12 nmol/L, assessment of free or bioavailable testosterone is recommended 7
Measurement Methods
- Bioavailable testosterone can be either measured directly (assayed BT) or calculated (calculated BT) 1
- Direct measurement is typically done using ammonium sulfate precipitation to separate SHBG-bound testosterone 1, 4
- Calculated bioavailable testosterone depends on the values of the association constants of total testosterone for SHBG and albumin, which can lead to variability in results 1
- For optimal calculation of bioavailable testosterone, the binding constants should be: SHBG (Ks) = 1.9 x 10^9 L/mol and albumin (Ka) = 2.45 x 10^4 L/mol 1
Common Pitfalls
- Many men receive testosterone therapy without proper diagnosis - up to 25% of men who receive testosterone therapy do not meet the criteria to be diagnosed as testosterone deficient 3
- Nearly half of men placed on testosterone therapy do not have their testosterone levels checked after therapy commences 3
- Sampling time is critical - measurements should be carried out in the morning, before 11:00 h, to account for diurnal variation 6
- In samples with equivocal concentration (7.0-12.5 nmol/L), at least one more estimate should be obtained to account for significant intra-individual variability 6