Most Useful Testosterone Test for Baseline Measurement
Morning total testosterone measured between 8-10 AM on two separate occasions is the most useful baseline test for diagnosing hypogonadism in adult males. 1, 2, 3, 4, 5
Diagnostic Algorithm
Initial Testing Requirements
Measure fasting morning total testosterone (8-10 AM) on two separate days to confirm persistent hypogonadism, as single measurements are insufficient due to assay variability and diurnal fluctuation 1, 2, 3, 4, 5
Both measurements must be below 300 ng/dL to establish biochemical hypogonadism 2, 3, 5
Morning timing is critical because testosterone levels naturally peak in the morning and decline throughout the day 2, 3
When to Add Free Testosterone Measurement
After confirming low total testosterone on two occasions, measure free testosterone by equilibrium dialysis in these specific situations: 1, 3, 4, 5
- Total testosterone is borderline (280-350 ng/dL) 6
- Suspected sex hormone-binding globulin (SHBG) abnormalities 1, 4, 5
- Obesity is present (increased aromatization affects SHBG) 2, 3
- Diabetes or metabolic syndrome 3
Additional Confirmatory Testing
Once low testosterone is confirmed, measure luteinizing hormone (LH) and follicle-stimulating hormone (FSH) to distinguish: 1, 3, 7, 4, 5
- Primary hypogonadism: Low testosterone with elevated LH/FSH (testicular failure) 1, 3, 7
- Secondary hypogonadism: Low testosterone with low or low-normal LH/FSH (hypothalamic-pituitary dysfunction) 1, 3, 7
This distinction has critical treatment implications, particularly for fertility preservation 3, 5
Diagnostic Thresholds and Clinical Interpretation
Total Testosterone Levels
- <230 ng/dL: Usually benefits from testosterone replacement therapy in symptomatic men 2
- 231-346 ng/dL: Gray zone where a 4-6 month trial may be considered after careful risk-benefit discussion 2, 3
- >350 ng/dL: Typically does not require replacement therapy 2, 6
Performance Characteristics of Total Testosterone
- Total testosterone <280 ng/dL has 91.0% sensitivity and 73.7% specificity for predicting low free testosterone 6
- Total testosterone must exceed 350-400 ng/dL to reliably predict normal free testosterone 6
- Total testosterone between 280-350 ng/dL is not sensitive enough to reliably exclude hypogonadism 6
Common Pitfalls to Avoid
Never diagnose hypogonadism based on a single testosterone measurement - assay variability and diurnal fluctuation require confirmation 2, 3, 4, 5
Never measure testosterone at random times of day - non-morning measurements are unreliable and lead to misdiagnosis 2, 3
Never skip the repeat measurement - approximately 20-30% of men with one low value will have normal levels on repeat testing 2, 3
Never start treatment without confirming both low testosterone AND symptoms - up to 25% of men receiving testosterone therapy do not meet diagnostic criteria 2, 3
Never use symptoms alone to diagnose hypogonadism - screening questionnaires lack specificity and must be confirmed biochemically 2, 3
Special Populations Requiring Free Testosterone
In men with obesity, diabetes, or conditions affecting SHBG, free testosterone measurement becomes essential even when total testosterone appears borderline: 2, 3, 5