Understanding Testosterone Types in Men
Total testosterone measures all the testosterone in your blood, free testosterone measures only the testosterone that's not stuck to proteins (about 2-3% of the total), and bioavailable testosterone includes both free testosterone plus testosterone loosely attached to a protein called albumin—think of it like measuring all your money (total), just the cash in your pocket (free), or cash plus money in your wallet that's easy to spend (bioavailable). 1
Breaking Down the Three Types
Total Testosterone
- This is the sum of all testosterone floating around in your blood 1
- Most of it (about 98%) is stuck to two proteins: SHBG (sex hormone-binding globulin) and albumin 2
- It's like measuring all the money you own, whether it's locked in a safe or in your pocket
- This is usually the first test doctors order because it's easier and cheaper to measure 1
Free Testosterone
- This is the unbound testosterone—only 0.5-3% of your total testosterone 1, 3
- It's not attached to any proteins and can enter cells right away to do its job 2
- Think of it as the cash in your pocket—immediately available to use
- Free testosterone often matches better with actual symptoms (like low energy or sex drive) than total testosterone does 2
Bioavailable Testosterone
- This includes free testosterone PLUS testosterone loosely bound to albumin 2
- The albumin-bound testosterone can easily break free and enter cells, especially in organs like the liver and brain 3
- It's like having cash in your pocket plus money in your wallet—both are easy to access and spend
- SHBG-bound testosterone is NOT included because it's stuck too tightly 2
When Each Test Matters
Use Total Testosterone When:
- You're doing the initial screening for low testosterone 1
- The person is young and healthy without obesity or diabetes 1
- Results are clearly very low or very high
Use Free or Bioavailable Testosterone When:
- Total testosterone is borderline (close to the lower limit of normal) 1
- The person has conditions that mess with SHBG levels 1, 3:
- Obesity (lowers SHBG, making total testosterone look falsely low)
- Diabetes (lowers SHBG)
- Older age (raises SHBG, making total testosterone look falsely high)
- Thyroid problems (can raise or lower SHBG)
- Liver disease (affects SHBG)
Important Measurement Details
Common pitfall: Many local labs use "analog" or "direct" immunoassays for free testosterone—these are extremely inaccurate and should NOT be used 2, 1. The accurate methods are:
- Equilibrium dialysis (gold standard but expensive) 2
- Calculation using total testosterone, SHBG, and albumin levels 2, 1
Another pitfall: Different labs can give wildly different results for the same blood sample—values can range from 45 to 365 ng/dL for the exact same person 3. Use labs certified by the CDC Hormone Standardization Program when possible 3.
Real-World Example
Imagine two 60-year-old men both have total testosterone of 320 ng/dL (just barely in the normal range):
- Man A is obese with diabetes → His SHBG is low → His free testosterone might actually be normal → He may not need treatment
- Man B is thin and healthy → His SHBG is normal or high → His free testosterone is likely low → He probably has real testosterone deficiency
This is why measuring free or bioavailable testosterone clarifies the picture when total testosterone is borderline 1, 3.
The Bottom Line for Understanding
The key difference is availability: Total testosterone is everything you have, free testosterone is what's immediately ready to work, and bioavailable testosterone is what's ready to work plus what can quickly become available. When proteins like SHBG change (from age, weight, or disease), total testosterone can be misleading—that's when free or bioavailable testosterone gives you the real story 1, 3.