Your doctor says your total testosterone is "normal." But you still have every symptom of low T. How is that possible? Often the answer lies in understanding the difference between total testosterone and free testosterone — and why only one of those numbers actually matters at the cellular level.

Total Testosterone: What It Measures

When a standard blood test reports "testosterone," it typically measures total testosterone — the sum of all testosterone in your blood regardless of whether it's biologically active or not.

In reality, testosterone in the blood exists in three forms:

  • SHBG-bound testosterone (~45–65%): Tightly bound to sex hormone-binding globulin. Biologically inactive — cannot enter cells.
  • Albumin-bound testosterone (~33–54%): Loosely bound to albumin. Weakly active — can be released by tissues.
  • Free testosterone (~1–3%): Unbound. Fully biologically active — directly enters cells and activates testosterone receptors.

Total testosterone adds all three together. But your cells only "see" the free fraction (and to a lesser extent, albumin-bound).

Free Testosterone: What It Measures

Free testosterone is the unbound, immediately bioavailable portion of circulating testosterone. It's the testosterone that:

  • Enters muscle cells and stimulates protein synthesis
  • Crosses the blood-brain barrier to affect mood and cognition
  • Acts on androgen receptors throughout the body
  • Can be converted to DHT (by 5-alpha reductase) and estradiol (by aromatase)

Even though free T represents only 1–3% of total testosterone, it's the fraction that drives virtually all of testosterone's physiological effects.

Why Normal Total T Doesn't Always Mean Normal Free T

This is where many men — and many physicians — get misled. SHBG levels vary enormously between individuals and change with:

  • Age: SHBG rises approximately 1–2% per year after age 40, steadily reducing free T even as total T remains stable
  • Liver disease, hyperthyroidism: Increase SHBG
  • Obesity, insulin resistance, hypothyroidism: Decrease SHBG
  • Estrogen levels (including medication): Increase SHBG

A man with total T of 500 ng/dL and high SHBG (e.g., 65 nmol/L) may have a free T of only 6–8 pg/mL — clearly in the low range. His total T looks normal, but his cells are experiencing functional testosterone deficiency.

Calculate your free testosterone

Use the Vermeulen formula with your lab values.

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How Free Testosterone Is Measured

There are three methods, with very different accuracy:

1. Equilibrium Dialysis (Gold Standard)

The most accurate method. A blood sample is dialyzed against a buffer; the concentration in the dialysate reflects true free T. Expensive (~$100–300), not widely available outside specialized labs.

2. Calculated Free Testosterone (Vermeulen Formula)

Calculates free T from total T, SHBG, and albumin (assuming standard albumin of 4.3 g/dL). Correlates well with dialysis. This is the method used by our Free T Calculator and by most clinical guidelines.

3. Direct Immunoassay

Many standard labs offer "direct free testosterone" by immunoassay. The Endocrine Society explicitly does not recommend this method due to poor accuracy and high variability between assays. If your lab reports free T by immunoassay, ask for SHBG to calculate it properly instead.

What Is Bioavailable Testosterone?

Bioavailable testosterone = free testosterone + albumin-bound testosterone. It represents the total fraction that tissues can access. The ammonium sulfate precipitation method is the standard assay; it's more commonly available than dialysis and is a reasonable alternative to calculated free T.

Normal range for bioavailable testosterone in men: approximately 130–300 ng/dL (varies by age).

What Do Endocrinologists Recommend?

Current Endocrine Society guidelines (2018) recommend:

  • Measure total testosterone first
  • If total T is near the lower limit of normal (300–400 ng/dL) OR if clinical suspicion is high, also measure SHBG and calculate free testosterone using the Vermeulen formula
  • Do not use direct immunoassay free T measurements for clinical decisions
  • Diagnosis requires two below-normal morning measurements

Practical Takeaway

If your total testosterone is in the 300–500 ng/dL range and you have symptoms, request SHBG to calculate your free T. You may find that your functional testosterone is lower than your total T suggests — which explains your symptoms and may inform treatment decisions.

FAQ

Yes, this is common, especially in older men. High SHBG can bind large amounts of testosterone, leaving low free T despite a "normal" total T reading.
The gold standard is equilibrium dialysis. The Vermeulen formula (calculating free T from total T and SHBG) is the most practical and widely used alternative. Direct immunoassay free T tests are less accurate and generally not recommended.
Bioavailable testosterone is free testosterone plus albumin-bound testosterone. Unlike SHBG-bound testosterone, albumin-bound T can be released by tissues and is partially biologically active.
Medical Disclaimer: This article is for educational purposes only. Consult a licensed healthcare provider for interpretation of your specific lab results and medical decisions.