What is the difference between measuring total testosterone and free testosterone in clinical practice?

Total Testosterone vs Free Testosterone in Clinical Practice

Primary Recommendation

Total testosterone should be the initial screening test for testosterone deficiency, measured on two separate early morning occasions with a threshold below 300 ng/dL, but free testosterone measurement becomes essential when total testosterone is borderline (280-400 ng/dL) or when sex hormone-binding globulin (SHBG) abnormalities are suspected. 1

When to Measure Total Testosterone

• Total testosterone is the first-line test for evaluating suspected testosterone deficiency and should be drawn between 8 AM and 10 AM on at least two separate occasions to account for diurnal variation 1
• The diagnosis of testosterone deficiency requires both total testosterone consistently below 300 ng/dL AND the presence of clinical symptoms or signs 1
• Total testosterone alone is adequate for initial screening in most men, as it has 91% sensitivity for detecting low free testosterone when using a threshold below 280 ng/dL 2

Clinical Context for Total Testosterone Measurement

Measure total testosterone even without symptoms in patients with: 1

• Unexplained anemia
• Bone density loss
• Diabetes
• History of chemotherapy or testicular radiation
• HIV/AIDS
• Chronic narcotic use
• Male infertility
• Pituitary dysfunction
• Chronic corticosteroid use

When Free Testosterone Measurement is Essential

Free testosterone measurement is mandatory in three specific clinical scenarios: 1, 3, 2

1. Borderline Total Testosterone Values

• When total testosterone falls between 280-400 ng/dL, total testosterone measurement lacks sufficient sensitivity (only 91% at <280 ng/dL) and specificity (only 73.7%) to reliably diagnose or exclude hypogonadism 2
• Total testosterone must exceed 350-400 ng/dL to reliably predict normal free testosterone 2

2. Suspected SHBG Abnormalities

• In men with obesity: Low total testosterone is frequently due to low SHBG rather than true hypogonadism; free testosterone by equilibrium dialysis is essential to distinguish between the two 1
• Obesity increases aromatization of testosterone to estradiol in adipose tissue, which can suppress luteinizing hormone and lower both total and free testosterone 1
• High SHBG states: When SHBG is elevated (thyroid disorders, aging, liver disease) but free testosterone is normal, testosterone replacement is NOT indicated 4

3. Discordant Clinical Picture

• When symptoms strongly suggest hypogonadism but total testosterone is near the lower limit of normal (280-350 ng/dL) 1, 3

Measurement Methodology Matters

Total Testosterone

• Liquid chromatography-tandem mass spectrometry (LC-MS/MS) is preferred over immunoassays for accuracy 3
• Use the same laboratory and same methodology for serial measurements 1

Free Testosterone

• Equilibrium dialysis is the gold standard but is too complex for routine clinical use 5, 6
• Calculated free testosterone (using total testosterone, SHBG, and albumin) is the recommended practical alternative 1, 5
• Direct analog immunoassays for free testosterone are unreliable and should be avoided 5, 6
• Free testosterone values from calculation are approximately 8 times higher numerically than analog immunoassay values, so different reference ranges must be applied 7

Critical Pitfall: High SHBG with Normal Free Testosterone

If calculated free testosterone is truly normal despite elevated SHBG, testosterone replacement therapy is NOT indicated 4

In this scenario: 4

• Normal free testosterone indicates adequate bioavailable testosterone for tissue effects
• Symptoms should be attributed to other causes
• Investigate alternative etiologies: thyroid function, liver function, other metabolic conditions
• Repeat assessment in 6-12 months to ensure stability
• Address modifiable factors affecting SHBG (weight management, thyroid optimization)

Diagnostic Algorithm

1. Initial screening: Two early morning (8-10 AM) total testosterone measurements 1

   • If both >400 ng/dL: Hypogonadism unlikely
   • If both <280 ng/dL with symptoms: Hypogonadism likely, proceed to LH/FSH 1
   • If 280-400 ng/dL OR obesity present: Proceed to step 2

2. Measure free testosterone (calculated), SHBG, and albumin 1, 3

   • If free testosterone low: Confirm hypogonadism, measure LH/FSH 1
   • If free testosterone normal: Hypogonadism excluded, investigate other causes 4

3. Measure LH and FSH to distinguish primary vs secondary hypogonadism 1

   • Low/normal LH with low testosterone: Secondary hypogonadism (check prolactin, consider pituitary imaging) 1
   • Elevated LH with low testosterone: Primary hypogonadism 1

Key Differences in Clinical Utility

Total testosterone reflects the overall testosterone production and is influenced by binding proteins (40% bound to SHBG, 58% bound to albumin, 2% free) 8, 5

Free testosterone represents the metabolically active fraction that is immediately available to tissues and is the more physiologically relevant measurement when binding protein abnormalities exist 5, 6

The free hormone hypothesis suggests that only unbound hormone can enter cells and exert biological effects, making free testosterone theoretically superior, though this remains somewhat controversial 5