Bioavailable Testosterone in Adult Males

Definition and Clinical Significance

• The normal percentage of bioavailable testosterone in adult males is approximately 33-50% of total testosterone, according to the Human Reproduction Update 1
• A threshold of approximately 0.78 nmol/L has been suggested as the lower limit of normal for bioavailable testosterone in adult men, as reported by the Human Reproduction Update 1

Measurement and Diagnosis

• Morning measurements are critical as testosterone levels naturally fluctuate throughout the day, with highest levels typically occurring in the morning 2
• The diagnosis of testosterone deficiency requires at least two separate morning measurements showing consistently low levels 2
• 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 2

Special Considerations

• Bioavailable testosterone measurement is particularly useful in men with conditions that affect protein binding, as noted by the Human Reproduction Update 1
• Bioavailable testosterone measurement is also useful in obese men, as SHBG decreases with obesity, according to Praxis Medical Insights 2