What is the normal percentage of bioavailable testosterone in adult males?

Normal Bioavailable Testosterone Percentage in Adult Males

The normal percentage of bioavailable testosterone in adult males is approximately 33-50% of total testosterone. 1, 2

Understanding Testosterone Forms and Distribution

• Total testosterone in the bloodstream exists in three forms:

  • Tightly bound to Sex Hormone-Binding Globulin (SHBG): approximately 44-65% 2, 3
  • Loosely bound to albumin: approximately 33-50% 2, 3
  • Free (unbound): approximately 1-3% 2, 3

• Bioavailable testosterone refers to the combination of free testosterone and albumin-bound testosterone, as the albumin binding is weak enough that this testosterone is available to tissues 2

• Bioavailable testosterone can be measured through laboratory assays using ammonium sulfate precipitation techniques or calculated using formulas that account for total testosterone, SHBG, and albumin levels 2

Clinical Significance of Bioavailable Testosterone

• Bioavailable testosterone is considered a more sensitive indicator of testosterone activity than total testosterone, particularly in aging men 3

• Studies show that bioavailable testosterone decreases with age even when total testosterone levels remain within normal limits 3, 4

• A threshold of approximately 0.78 nmol/L (2.30-2.72 nmol/L) has been suggested as the lower limit of normal for bioavailable testosterone in adult men 2, 1

Measurement Considerations

• Morning measurements are critical as testosterone levels naturally fluctuate throughout the day, with highest levels typically occurring in the morning 5, 6

• The diagnosis of testosterone deficiency requires at least two separate morning measurements showing consistently low levels 5, 6

• When evaluating men with obesity, measurement of bioavailable testosterone is particularly important as obesity affects SHBG levels, potentially masking testosterone deficiency when only total testosterone is measured 5

Clinical Applications

• Bioavailable testosterone measurement is particularly useful in:

  • Older men (as SHBG increases with age) 3, 4
  • Obese men (as SHBG decreases with obesity) 5
  • Men with conditions that affect protein binding 1
  • Cases where total testosterone is borderline low (8-12 nmol/L or 230-350 ng/dL) 7

• When total testosterone levels fall in the "grey zone" between 8-12 nmol/L (230-350 ng/dL), assessment of bioavailable testosterone can help clarify whether a true testosterone deficiency exists 7

Common Pitfalls in Assessment

• Using inappropriate association constants when calculating bioavailable testosterone can lead to significant errors in estimation 2

• Failure to account for other hormones like androstenediol that compete for SHBG binding can affect bioavailable testosterone calculations 2

• Not considering age-related changes in bioavailable testosterone when interpreting results may lead to missed diagnoses of testosterone deficiency in older men 3, 4