Should free testosterone or total testosterone be tested first in patients with suspected hypogonadism (testosterone deficiency)?

Total Testosterone Should Be Tested First in Patients with Suspected Hypogonadism

Clinicians should use total testosterone as the initial diagnostic test for patients with suspected hypogonadism, with a level below 300 ng/dL as a reasonable cut-off to support the diagnosis of low testosterone. 1

Diagnostic Algorithm for Testosterone Deficiency

Initial Testing

• Measure morning total testosterone levels between 8 AM and 10 AM
• Repeat measurement on a separate occasion, also in early morning
• Both measurements should be conducted using the same laboratory with the same method/instrumentation

When to Add Free Testosterone Testing

Free testosterone should be measured in specific situations:

• When total testosterone is near the lower limit of normal
• When sex hormone-binding globulin (SHBG) abnormality is suspected
• In patients with obesity (who often have low SHBG levels)

Evidence Supporting Total Testosterone as First-Line Test

The American Urological Association (AUA) provides strong guidance on this matter, recommending total testosterone as the initial diagnostic test with a threshold of <300 ng/dL to support the diagnosis of testosterone deficiency 1. This recommendation carries a moderate strength rating with Grade B evidence level.

The diagnosis requires:

1. Consistently low total testosterone (<300 ng/dL) on at least two separate morning measurements
2. Presence of symptoms and/or signs consistent with testosterone deficiency

When Free Testosterone Testing Becomes Important

While total testosterone is the appropriate initial test, free testosterone measurement becomes crucial in specific scenarios:

• In patients with obesity, where low total testosterone may be due to decreased sex hormone-binding globulin rather than true testosterone deficiency 1
• When evaluating low total testosterone in men with obesity, morning serum free testosterone measurement becomes essential 1
• In men whose total testosterone is near the lower limit of normal 2, 3

Adjunctive Testing After Low Testosterone is Confirmed

Once low testosterone is confirmed, additional testing should include:

• Luteinizing hormone (LH) levels to determine if hypogonadism is primary or secondary 1
• Prolactin levels (if LH is low or low/normal) 1
• Follicle-stimulating hormone (FSH) for men interested in fertility 1

Common Pitfalls to Avoid

1. Single measurement error: Relying on a single testosterone measurement can lead to misdiagnosis. Always confirm with a second morning measurement 1.

2. Incorrect timing: Testosterone levels fluctuate throughout the day. Measurements should be taken in early morning when levels are typically highest 1.

3. Using questionnaires alone: Screening questionnaires should not replace laboratory testing as they have variable sensitivity and specificity 1.

4. Ignoring SHBG variations: In conditions that alter SHBG (like obesity), total testosterone may be misleadingly low while free testosterone remains normal 1. In these cases, free testosterone should be measured.

5. Using inappropriate thresholds: Total testosterone between 280-350 ng/dL is not sensitive enough to reliably exclude hypogonadism. Total testosterone must exceed 350-400 ng/dL to reliably predict normal free testosterone 4.

Quality Considerations for Testing

For accurate diagnosis:

• Use reliable assays for testosterone measurement
• For free testosterone, equilibrium dialysis is the preferred method 5
• Laboratory certification by an accuracy-based program (e.g., CDC's Hormone Standardization Program) is ideal 5

By following this evidence-based approach, clinicians can effectively diagnose testosterone deficiency while avoiding unnecessary testing or missed diagnoses.