- Total testosterone includes bound and free hormone; free testosterone is what's available to work.
- High SHBG can mask low free testosterone in a normal total-testosterone result.
- If total testosterone looks respectable but you're tired and recovering poorly, check SHBG and free testosterone.
- Calculated free testosterone is reliable and less expensive than direct measurement.
- Training type and metabolic health are the strongest levers on the free-to-total ratio.
Quick Answer
Your total testosterone result doesn't tell the whole story. Only about 2% of testosterone circulates freely; the rest is bound to carrier proteins like SHBG or albumin. If you have high SHBG, most of your testosterone is locked up and unavailable to your muscles, brain, and recovery systems: even if your total testosterone looks normal. This is why two people with identical total testosterone can experience completely different symptoms and training adaptations. Free testosterone is the active hormone; total is just the inventory.
Why This Matters for Athletes
If you're training hard and your total testosterone is normal but you're struggling with energy, slow recovery, or weak muscle gains, your free testosterone might be the culprit.
High SHBG, hidden low free testosterone: Endurance athletes often have higher SHBG, which ties up testosterone. You can have total testosterone of 15 nmol/L (normal) and free testosterone of 0.3 nmol/L (low).[3] You'll feel the low number, not the respectable-looking total.
Recovery and protein synthesis: Only free testosterone can bind to androgen receptors in muscle, triggering protein synthesis and recovery.[5] Bound testosterone is physiologically inactive. Your muscles don't care about total testosterone; they respond to free testosterone.
Training adaptation: Strength training typically lowers SHBG slightly and increases total testosterone, increasing free hormone availability: a double win for muscle growth. High-volume endurance training may raise SHBG, reducing free hormone despite stable or even elevated total.[4] This is a key adaptation difference to understand if you're periodising or changing sports.
The Numbers Explained
Total testosterone reference ranges (adult men):[1] approximately 10–30 nmol/L depending on assay. (Women: 0.5–2.5 nmol/L.)
Free testosterone reference ranges: typically 180–700 pmol/L for men, or roughly 0.2–0.7 nmol/L depending on how your lab reports it. Women's ranges are much lower.
| Marker | What it tells you |
|---|---|
| Total testosterone | Overall hormone inventory; includes bound and free |
| Free testosterone | Hormone available to tissues; the biologically active fraction |
| SHBG | The carrier protein that affects the ratio of bound to free |
| Albumin | A secondary carrier protein (less important than SHBG) |
How to Interpret Your Results
Total normal, free normal: You're in good shape. Adequate hormone availability.
Total normal, free low: High SHBG is likely binding most of your testosterone. You may experience fatigue, slow recovery, or low libido despite normal-looking total. Check SHBG and metabolic markers (glucose, insulin).
Total high, free high: Plenty of hormone available. Check for over-training or inadequate recovery.
Total low, free low: Low hormone production. See your GP to investigate pituitary, testicular, or metabolic issues.
Total low, free normal: Unusual but possible with very low SHBG. Less common and less clinically relevant.
When to see a GP first: If you're experiencing symptoms of low testosterone (fatigue, low mood, weak recovery, low libido) alongside abnormal results, or if you're considering supplementation, consult your GP. They can rule out testicular, pituitary, or metabolic issues before investigating training or nutrition factors.
What Affects Your Free-to-Total Ratio
Training Type and Volume
Resistance training lowers SHBG slightly, increasing free testosterone relative to total. High-volume endurance training raises SHBG, lowering the free fraction. This is one of the biggest levers you control.
Calorie Intake
Undereating raises SHBG, reducing free hormone availability. Eating adequately for your training supports better ratios.
Insulin Sensitivity
Better metabolic health correlates with lower SHBG and thus higher free testosterone for a given total.[4] This is why metabolic fitness and testosterone often improve together.
Age
SHBG naturally increases slightly with age, lowering the free fraction even if total testosterone stays stable. This contributes to age-related drops in free hormone.
Liver and Metabolic Health
SHBG is produced by the liver. Liver stress or poor metabolic health (fatty liver, metabolic syndrome) can elevate SHBG and suppress free hormone.
Alcohol and Medications
Excess alcohol raises SHBG. Some medications (including statins in high doses and certain anti-seizure drugs) can affect SHBG production.
When to Retest
If your initial results are normal (both total and free), retest annually or after significant changes in training, weight, or symptoms. If results are abnormal or symptoms persist, retest after 6–8 weeks of changes (training adjustment, nutrition changes, etc.). Don't retest too quickly; hormonal adaptations take weeks to manifest.
FAQ
Can I increase my free testosterone without supplementation? Yes. The primary lever is improving the free-to-total ratio through training (resistance over endurance), adequate calorie intake, and metabolic health (glucose control, weight management). These changes take weeks to months.
Is calculated free testosterone accurate enough? For most people, yes. Calculated free testosterone (derived from total, SHBG, and albumin) correlates well with measured free testosterone. Direct measurement (equilibrium dialysis) is gold-standard but expensive and rarely changes clinical management.
Why do some athletes have high total but low free testosterone? High SHBG. This is common in distance runners and other endurance specialists. The adaptation makes sense physiologically, but it means less recovery signal despite high total hormone. Understanding this can guide training adjustments.
If my total is normal but free is low, should I supplement testosterone? Not without investigation first. Supplementing testosterone without addressing the underlying cause (usually high SHBG due to training patterns, calorie deficit, or metabolic stress) often backfires. Work with your GP to identify the cause.
How quickly do free and total testosterone change with training shifts? Total testosterone can shift within days of training load changes, but free testosterone usually takes 2–3 weeks to stabilise as SHBG adjusts. Longer-term adaptations take 6–8 weeks.
