The Three Fractions of Testosterone (Simplified)
Most testosterone in your bloodstream isn't actually available for your body to use. It's bound — attached to proteins — which renders it biologically inactive.
Your total testosterone measurement includes all three fractions:
| Fraction | Bound to | Biologically Active? | Approximate % |
|---|---|---|---|
| SHBG-bound testosterone | Sex hormone-binding globulin (tight bind) | ❌ Not usable | 44–65% |
| Albumin-bound testosterone | Albumin (loose bind) | ✅ Weakly available | 33–50% |
| Free testosterone | Nothing | ✅ Fully active | 1–4% |
Total testosterone = all three fractions added together.
Free testosterone = just the 1–4% that's immediately bioavailable.
Bioavailable testosterone = free T + albumin-bound T combined (the fraction your tissues can actually access).
The critical insight: your cells use free testosterone. That's what binds to androgen receptors, drives muscle protein synthesis, regulates libido, mood, cognition, and energy. SHBG-bound testosterone is essentially a reservoir — biologically inert until SHBG releases it.
When SHBG is elevated, it locks up more testosterone, leaving less in the free fraction. You can have a "normal" total testosterone of 450–550 ng/dL while your free testosterone is functionally in the basement.
Not sure if your labs are telling the whole story? Take the TRT Readiness Quiz →
Why Total Testosterone Alone Can Mislead You
Standard testosterone testing typically reports total T. Many conventional primary care doctors only order total T. And many clinic threshold decisions are based on total T alone.
The problem: total T doesn't tell you how much testosterone your body can actually use.
Here's a concrete example:
- Man A: Total T = 480 ng/dL. SHBG = 22 nmol/L (low-normal). Free T = 14.2 pg/mL (well within range). Likely feels fine.
- Man B: Total T = 480 ng/dL. SHBG = 61 nmol/L (elevated). Free T = 6.8 pg/mL (below 9 pg/mL functional threshold). Likely symptomatic.
Same total T. Dramatically different clinical picture.
Man B has what's sometimes called functional hypogonadism — lab values that look "normal" by conventional cutoffs but are consistent with testosterone deficiency at the tissue level. His symptoms (fatigue, low libido, brain fog, poor gym recovery) are real. His total T test is simply not measuring what drives them.
What Is SHBG and Why Does It Change Everything?
Sex hormone-binding globulin (SHBG) is a protein produced primarily by the liver. Its job is to bind sex hormones — primarily testosterone and estradiol — and regulate how much circulates freely.
SHBG levels vary enormously between men and change with:
- Age — SHBG rises ~1–2% per year after 40; men in their 60s and 70s often have SHBG 50–80% higher than men in their 30s
- Body weight — obesity and insulin resistance lower SHBG; lean, muscular men often have higher SHBG
- Thyroid function — hypothyroidism lowers SHBG; hyperthyroidism raises it
- Alcohol — moderate/heavy use raises SHBG significantly
- Certain medications — anticonvulsants, some antiretrovirals raise SHBG
- Liver function — liver disease can dramatically alter SHBG production
This is why two men with identical total T can have completely different symptom profiles and completely different free T levels. SHBG is the variable that conventional "testosterone is normal" conclusions almost always miss.
For a deep dive on elevated SHBG specifically — including the 8 most common causes, interventions to lower it, and how to use SHBG to guide TRT decisions — see: High SHBG and Low Free Testosterone: Why "Normal" Labs Might Be Missing the Actual Problem.
How Free Testosterone Is Measured (and Why Labs Differ)
Free testosterone can be measured or calculated. The method matters more than most people realize.
Direct Assay (Most Common in Labs)
The most widely used lab method is a direct radioimmunoassay (RIA). It's cheap and fast. It's also notoriously inaccurate for free testosterone. Multiple studies comparing direct assay to equilibrium dialysis (the gold standard) have found the direct assay systematically underestimates free T — and the margin of error is wide enough to produce meaningless results for clinical decision-making.
Calculated Free T (Vermeulen Formula)
Many clinics and some labs now report free testosterone calculated using the Vermeulen formula — a validated mathematical model that uses total T, SHBG, and albumin to estimate free T. This approach is significantly more accurate than direct assay and correlates well with equilibrium dialysis results. If you have total T and SHBG values, you can calculate free T yourself using an online Vermeulen calculator.
Equilibrium Dialysis (Gold Standard)
This is the most accurate measurement method but rarely used outside research settings due to cost and turnaround time.
Bottom line: Ask your provider whether your free T was measured (direct assay) or calculated (Vermeulen). Calculated free T from a quality SHBG measurement is more trustworthy than a cheap direct assay.
Free Testosterone Reference Ranges by Age
| Age Group | Total T Normal Range | Free T Normal Range (Calculated) | Free T Functional Threshold |
|---|---|---|---|
| 20–29 | 400–1,200 ng/dL | 16–31 pg/mL | Below 12–14 pg/mL symptomatic |
| 30–39 | 350–1,100 ng/dL | 14–28 pg/mL | Below 10–12 pg/mL symptomatic |
| 40–49 | 300–950 ng/dL | 11–24 pg/mL | Below 9–10 pg/mL symptomatic |
| 50–59 | 250–850 ng/dL | 9–20 pg/mL | Below 8–9 pg/mL symptomatic |
| 60–69 | 200–750 ng/dL | 7–17 pg/mL | Below 7 pg/mL likely symptomatic |
| 70+ | 200–600 ng/dL | 5–14 pg/mL | Context-dependent |
Key nuance: "Normal range" is a population statistic, not a personal threshold. A 45-year-old with free T at 9.2 pg/mL is technically "in range" but may be symptomatic if his personal baseline was 18 pg/mL a decade ago. This is why symptom context matters as much as lab numbers.
For age-referenced total testosterone ranges by decade — including why draw timing dramatically affects results — see Testosterone Levels by Age: Normal Ranges Chart for Men.
When Total T Is Normal but Free T Is Low: The Functional Hypogonadism Case
This is the scenario that conventional primary care frequently misses — and that TRT-aware clinicians and specialty practices identify most often.
The pattern: Total testosterone in the 350–600 ng/dL range. Free testosterone calculated at or below 9–11 pg/mL. SHBG elevated (typically 50–80+ nmol/L). Symptoms consistent with hypogonadism.
Why it gets missed:
- Only total T is ordered
- Total T is technically "normal"
- Doctor concludes testosterone is fine
- Symptoms attributed to depression, aging, stress, or "just life"
What actually drives symptoms: Research on hypogonadal symptoms (low libido, fatigue, poor concentration, erection quality, muscle loss) consistently finds stronger correlations with free testosterone and bioavailable testosterone than with total testosterone alone. The tissue-level experience of testosterone is driven by what can actually bind androgen receptors — and that's free T.
If you're symptomatic with a total T in the 350–550 ng/dL range, requesting SHBG and a calculated free T is the correct next diagnostic step before concluding your testosterone is fine.
If this scenario sounds familiar — normal-ish total T, real symptoms, dismissive doctor — the quiz helps map your pattern and clarify next steps. Start the quiz →
Which Number Matters More for Symptoms?
| Scenario | Which Number Matters Most |
|---|---|
| Total T above 600 ng/dL, no symptoms | Total T sufficient; symptoms likely non-hormonal |
| Total T below 300 ng/dL, symptomatic | Total T is clearly low — free T confirms severity |
| Total T 350–600 ng/dL, symptomatic | Free T + SHBG is the critical clarifying step |
| On TRT, total T looks fine but still symptomatic | Free T + SHBG + E2 — likely SHBG-driven or crashed E2 |
| Normal total T, no SHBG test, symptomatic | Incomplete panel — SHBG + free T needed to interpret |
When total T is clearly high (600+ ng/dL), free T is almost always adequate. When total T is clearly low (sub-300 ng/dL), free T confirms the deficiency. The diagnostic gray zone — 350–600 ng/dL — is where free testosterone becomes the determinative variable.
Bioavailable Testosterone: The Third Number
Some labs and clinics report "bioavailable testosterone" as a separate value. This is the sum of free testosterone + albumin-bound testosterone.
Since albumin-bound testosterone can dissociate from albumin relatively easily (unlike the tighter SHBG binding), it's considered partially accessible to tissues. In practice, bioavailable T is most useful when SHBG is significantly elevated (making free T very low) or when comparing lab reports from different facilities. For most decisions, if you have SHBG and total T, calculating free T via Vermeulen is equivalent to or better than a direct bioavailable T assay.
What to Do If Your Total T Is Normal but You're Still Symptomatic
- Get SHBG and a calculated free T — the most important single next step. Ask your provider or use an at-home lab service that includes SHBG (LabCorp, Quest, LetsGetChecked, Marek Health, etc.).
- Calculate free T using the Vermeulen formula — if your lab only provides direct assay free T, run the Vermeulen calculation yourself using total T, SHBG, and albumin (albumin defaults to 4.3 g/dL if not measured).
- Get a morning draw — testosterone peaks 8–10 AM and drops significantly by afternoon. An afternoon draw can produce a result 20–30% lower than a true fasting morning value.
- Evaluate SHBG causes — if SHBG is elevated, investigate why before pursuing TRT. Thyroid dysfunction, alcohol, liver function, and significant weight loss are common addressable causes.
- Get LH and FSH — these pituitary hormones tell you whether the problem is in the testis (primary hypogonadism — high LH/FSH) or in the signaling (secondary — low/normal LH/FSH). This distinction matters enormously for treatment decisions.
- Review your full symptom picture — low free testosterone can cause: low libido, fatigue, poor erection quality, reduced gym recovery, brain fog, mood instability, disturbed sleep, and reduced motivation. If you have 4+ of these alongside low-ish total T and low free T, that's a coherent pattern worth addressing.
You don't need to decode this alone. Take the TRT Readiness Quiz → — it walks through your symptoms, labs, and history to give you a clear picture of where you stand.
For a complete guide to what labs to order before any testosterone treatment decision, see TRT Bloodwork Panel: What to Test Before, During, and After Testosterone Therapy.
Free Testosterone and TRT: Why It Matters During Treatment Too
If you're already on testosterone therapy, free testosterone matters even more — not less.
- SHBG-driven suboptimal response: You're on TRT, total T reads 700 ng/dL, but SHBG is 65 nmol/L. Your free T is still only 12 pg/mL. Switching to more frequent, smaller doses (twice-weekly or daily SubQ microdosing) can lower SHBG over time and improve free T without raising total T further.
- Oral TRT's SHBG advantage: Oral testosterone formulations (Jatenzo, Kyzatrex) pass through the liver and suppress SHBG by 30–50%. For men with very high SHBG who struggle to achieve adequate free T on injections, oral TRT's SHBG-lowering mechanism can produce dramatically better free T outcomes.
- The "good labs but bad symptoms" puzzle: Total T looks great (700–900 ng/dL), but the patient still reports fatigue, low libido, or poor recovery. Free T, SHBG, and estradiol (E2) are the three labs to pull first — and crashed E2 from aromatase inhibitor overuse is frequently the culprit.
If you're on TRT and it's not working as expected, see Why Isn't My TRT Working? The 9 Most Common Reasons Protocol Results Stall.
Common Mistakes When Interpreting Testosterone Labs
- Treating total T alone as the verdict — without SHBG, total T gives you a rough directional signal but not a complete picture. It's like checking your bank balance without knowing how much is already owed.
- Using direct assay free T without cross-checking — direct assays are often 15–40% below true value. If your free T was measured by direct assay, calculate it via Vermeulen before acting on that number.
- Getting a random or afternoon draw — testosterone has strong diurnal variation. One low result from a 3 PM appointment is not a diagnosis.
- Looking only at "in range" vs. "out of range" — reference ranges are population distributions, not individual benchmarks. The bottom 5% of "normal" is still below average for a reason.
- Ignoring SHBG when evaluating TRT response — if you're on TRT and total T looks fine but symptoms persist, checking SHBG and calculated free T should always be the next move before changing dose or adding compounds.
Key Takeaways
- Total testosterone includes all three fractions — SHBG-bound (inactive), albumin-bound (partially available), and free T (fully active, ~1–4%). Your cells use free testosterone.
- Two men with identical total T of 480 ng/dL can have dramatically different free T levels based on SHBG — one fully functional, one symptomatic.
- The diagnostic gray zone is total T 350–600 ng/dL — this is where free T and SHBG become the determinative variables, and where most cases of functional hypogonadism are missed.
- Direct assay free T from most labs is unreliable — calculated free T via the Vermeulen formula (using total T + SHBG) is significantly more accurate.
- On TRT, free T matters more than ever — high SHBG can keep free T suboptimal even at good total T levels, and injection frequency or oral TRT formulation can be used to address this.
Want a more personalized read on your situation? Take the quiz → — it takes about 4 minutes and factors in symptom severity, lifestyle, lab patterns, and treatment preferences.