Total Testosterone versus Free Testosterone in Clinical Evaluation
The measurement of free testosterone is essential when alterations in binding protein concentrations are suspected or when total testosterone levels are borderline, as it provides a more accurate assessment of testosterone deficiency than total testosterone alone. 1
Understanding the Difference
- Total testosterone represents the sum of all testosterone in the blood, including both bound (98%) and unbound (2%) forms 2, 3
- Free testosterone represents only the unbound, biologically active portion of testosterone that is available to tissues 4
- Testosterone in plasma is predominantly (98%) bound to sex hormone-binding globulin (SHBG) and albumin, with only about 2% existing in the free form 2, 3
- According to the free hormone theory, free testosterone measurement may be more clinically relevant than total testosterone for assessing true androgenic status 4
Diagnostic Approach
- The American Urological Association recommends using a total testosterone level below 300 ng/dL as the cut-off to support the diagnosis of testosterone deficiency 5, 6
- Diagnosis requires two separate morning testosterone measurements (between 8-10 AM), both showing levels below 300 ng/dL 5, 6
- Clinical diagnosis of testosterone deficiency is only made when patients have both low total testosterone levels AND symptoms/signs of hypogonadism 5, 6
When to Measure Free Testosterone
- Free testosterone should be measured when:
- Alterations in binding protein concentrations are suspected 1
- Total testosterone levels are borderline (slightly above or below the lower limit of normal) 1
- Normal SHBG with reduced total morning testosterone is observed, to determine if true testosterone deficiency exists 7
- Obesity is present, as this can lead to low total testosterone with normal SHBG due to increased aromatization of testosterone to estradiol in adipose tissue 7
Measurement Methods
- Free testosterone measurement is challenging in clinical practice 4, 8
- The gold standard method is equilibrium dialysis, but this is too complex for routine clinical laboratories 4, 8
- Calculated free testosterone (CFT) using various equations is more practical but has limitations:
- Free androgen index is not recommended for men due to inaccuracy at extremes of SHBG concentration 4
Clinical Impact of Free vs. Total Testosterone Measurement
- Routine assessment of free testosterone allows for more accurate diagnosis of functional hypogonadism, especially in men with sexual symptoms 10
- In one study, total testosterone determination misdiagnosed hypogonadism in 8.4% of men with sexual symptoms and 2% with infertility 10
- In subjects with borderline total testosterone, only 24.7% had hypogonadism confirmed by free testosterone levels 10
- Free testosterone levels correlate better with clinical parameters of androgen deficiency (age, hematocrit, gonadotropins, BMI, comorbidities) than total testosterone levels 10
Best Practices for Testosterone Assessment
- Measure total testosterone in the early morning (8-10 AM) due to diurnal variation 7, 6
- Use a reliable assay such as liquid chromatography-tandem mass spectrometry in a certified laboratory 8, 1
- Obtain at least two fasting morning samples to confirm diagnosis 8
- When total testosterone results are equivocal, incorporate free testosterone measurement into the investigation 9
- For free testosterone, ideally use equilibrium dialysis method or validated calculation formulas 8
Common Pitfalls to Avoid
- Relying on a single testosterone measurement instead of two separate measurements 6
- Diagnosing testosterone deficiency based solely on laboratory values without considering clinical symptoms 6
- Using free androgen index in men, especially with extreme SHBG concentrations 4
- Failing to consider free testosterone when total testosterone is borderline or when SHBG alterations are suspected 1
- Using screening questionnaires alone for identifying candidates for testosterone therapy 7