Difference Between Free and Total Testosterone and Which to Follow for Hypogonadism
Start with morning total testosterone (drawn between 8-10 AM) as your initial test, but you must also measure free testosterone (by equilibrium dialysis or calculated from total testosterone, SHBG, and albumin) when total testosterone is borderline (near 300 ng/dL) or when conditions that alter SHBG are present—particularly in men with obesity, diabetes, aging (>60 years), or thyroid disease. 1, 2
Understanding the Key Differences
Total testosterone represents the sum of all testosterone in circulation: unbound (free), loosely bound to albumin, and tightly bound to sex hormone-binding globulin (SHBG). Only 0.5-3% circulates as unbound "free" testosterone. 1, 3
Free testosterone is the biologically active, unbound fraction that can enter cells and exert androgenic effects. The term "bioavailable testosterone" includes both free testosterone and the albumin-bound fraction, which dissociates readily. 1
The Critical Clinical Problem
Here's the trap that catches many clinicians: Men with obesity and hypogonadism frequently have low SHBG concentrations, which artificially lowers total testosterone while free testosterone remains normal. Conversely, aging men often have elevated SHBG, which can maintain normal total testosterone despite frankly low free testosterone levels. 4
This creates two scenarios where total testosterone alone misleads you:
- Low total testosterone with normal free testosterone (common in obesity): These men don't need testosterone replacement 4
- Normal total testosterone with low free testosterone (common in aging, especially >60 years): These men have true hypogonadism that you'll miss if you only check total testosterone 5, 6
Diagnostic Algorithm
Step 1: Initial Screening
- Measure fasting morning total testosterone (8-10 AM) using a reliable assay (liquid chromatography tandem mass spectrometry preferred) 1, 2, 7
- Normal range: 300-800 ng/dL in most laboratories 4, 8
Step 2: Confirm the Diagnosis
- Repeat total testosterone on a separate morning if initially low (30% of men with mildly low levels normalize on repeat testing) 1, 3
- Diagnosis requires two low measurements 1, 2, 8
Step 3: Measure Free Testosterone When:
You must measure free testosterone in these situations 1, 3, 2:
- Total testosterone is borderline (280-350 ng/dL)
- Obesity (very common—low SHBG falsely lowers total testosterone) 4
- Age >60 years (high SHBG can mask low free testosterone; 26.3% of ED patients >60 have normal total/low free testosterone) 5
- Diabetes mellitus
- Thyroid disease (hypothyroidism lowers SHBG; hyperthyroidism raises it)
- Chronic illness
- Hepatic disease
- HIV/AIDS
Step 4: How to Measure Free Testosterone
Gold standard: Equilibrium dialysis 1, 2
Acceptable alternative: Calculate free testosterone using validated formulas (Vermeulen method) from total testosterone, SHBG, and albumin 1, 3, 9
Never use: Analog immunoassays for free testosterone—these are inaccurate and affected by SHBG alterations 3
Treatment Decision Thresholds
In men with obesity specifically: Initiate testosterone replacement only when free testosterone by equilibrium dialysis is frankly low on at least 2 separate assessments 4. This is critical because obesity-related low total testosterone due solely to low SHBG does not represent true hypogonadism. 4
In general populations: The lower limit of normal for free testosterone by equilibrium dialysis is approximately 5-9 pg/mL (50-63 pg/mL in some labs), though this varies by laboratory 3, 6
Why Free Testosterone Matters More Clinically
Recent evidence demonstrates that free testosterone correlates better with clinical outcomes than total testosterone 6, 10:
- Free testosterone significantly associates with age, hematocrit, gonadotropins, gynecomastia, BMI, number of comorbidities, erectile dysfunction, and low libido
- Total testosterone only associates with BMI and low libido 6
- Using total testosterone alone to classify hypogonadism results in misclassification in 42% of cases compared to bioavailable testosterone 9
Common Pitfalls to Avoid
Don't skip free testosterone in older men: In patients >60 years with ED, 17.2% have normal total testosterone but low free testosterone—you'll miss these cases entirely 5
Don't use analog free testosterone assays: These are widely available but inaccurate 3
Don't diagnose during acute illness: Testosterone levels drop transiently during acute or subacute illness 1
Don't forget week-to-week variability: Testosterone levels can fluctuate substantially; some men test hypogonadal one week and normal the next, emphasizing the need for repeat testing 9
Don't treat low total testosterone in obese men without confirming low free testosterone: This is inappropriate therapy 4
Additional Workup After Confirming Low Testosterone
Once you've confirmed hypogonadism with appropriately low testosterone levels: