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Vermeulen formula (calculation of free testosterone)

Dr. Jonathan Apasu

Testosteron Experte von Adon Health

This is how free testosterone is calculated correctly

The Vermeulen formula is a scientifically established method for calculating free testosterone, that is, the portion that is biologically active in the body and can actually act on cell receptors. Unlike total testosterone, which is largely bound to proteins such as SHBG (sex hormone-binding globulin), free testosterone plays a crucial role in libido, muscle growth, energy, mood, and metabolism .

Since free testosterone is often not directly measured but estimated in standard laboratories, the Vermeulen formula is an important tool, both in medical practice and in modern hormone tests such as the TRT-Check from Adon Health.

Why is free testosterone so crucial?

Testosterone is present in the blood in three forms:

  • Bound to SHBG (approx. 60%) - biologically inactive
  • Bound to albumin (approx. 38%) - weakly bound, partially bioavailable
  • Free testosterone (approx. 1–3%) - biologically active, acts directly on the target cells

Only free testosterone (and to some extent albumin-bound testosterone) has the desired effect on libido, muscle strength, mood, bone density and metabolism.

A normal total testosterone level can be misleading; for example, if SHBG is too high , too little free testosterone is biologically available. This can explain why some men suffer from typical symptoms of testosterone deficiency despite "normal lab results."

What is the Vermeulen formula?

The Vermeulen formula is a mathematical equation for calculating free testosterone from the following laboratory values:

  • Total testosterone (in nmol/l or ng/dl)
  • SHBG (in nmol/l)
  • Albumin (optional, usually assumed to have a standard value of 43 g/l)

The formula is based on the law of mass action , i.e., on the binding equilibria between testosterone, SHBG and albumin.

In short:

The higher the SHBG level, the more testosterone is bound → less is freely available.

The calculation (simplified):

The formula calculates the proportion of testosterone that is not bound, i.e., the free, active fraction.

Click here to go to the computer.

When is the calculation useful?

  • In cases of symptoms of testosterone deficiency , but "normal" total testosterone levels
  • In cases of increased SHBG (e.g., due to age, hyperthyroidism, liver disease, diets)
  • During the evaluation prior to testosterone replacement therapy (TRT)
  • As part of a complete hormone profile for unfulfilled desire to have children, exhaustion or loss of libido
  • For monitoring progress during TRT, in order to better assess the effect.

💡 At Adon Health, the TRT check automatically calculates free testosterone based on the Vermeulen formula and includes medical interpretation if required.

What is considered normal?

The reference range for free testosterone (depending on the lab & source):

  • 0.245 – 0.785 nmol/l (approximately 7–23 ng/dl)
  • Values ​​below 0.243 nmol/l are often considered to require treatment, if corresponding symptoms are present.

Important: The assessment should always be made in the context of symptoms, age, SHBG, LH and other hormones, never based solely on a single value.

Advantages of the Vermeulen formula

Affordable and straightforward calculation using common laboratory values

High correlation with complex laboratory method (equilibrium dialysis)

Ideal for borderline cases, elevated SHBG or unclear symptoms

Enables informed decisions regarding treatment or further investigation.

Conclusion

The Vermeulen formula is a proven tool for reliably calculating free testosterone and thus better assessing the actual hormonal situation of men. Especially in cases of borderline total testosterone levels, elevated SHBG, or unclear symptoms, it provides important indications of whether a functional testosterone deficiency is present.

Men experiencing symptoms such as loss of libido, lack of motivation, muscle loss or low mood should not only have their total testosterone measured, but also their free testosterone calculated, ideally using the Vermeulen formula.

This helps to avoid misdiagnoses and to make a targeted, well-founded decision about TRT or lifestyle intervention .

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