Calculate Free Testosterone Instead of Trusting Total Testosterone Alone
Total testosterone alone is a weak screen. Free testosterone from total testosterone, SHBG and albumin shows the hormone your tissues can actually use.
Work out free testosterone from total testosterone, SHBG and albumin instead of trusting total testosterone alone. Total testosterone counts hormone that is bound to SHBG and to albumin, not just the free fraction that can move into cells, so two people with the same total can have very different amounts of usable hormone. The free testosterone calculator at HealthOS uses the Vermeulen 1999 equation to turn those three numbers into one value.
Free testosterone calculator: total testosterone, SHBG, albumin
You need three values from a hormone panel: total testosterone, sex hormone-binding globulin (SHBG) and albumin. The calculator returns free testosterone in nmol/L and ng/dL and a band for the relevant reference range. One common laboratory reference range for calculated free testosterone in adult men is about 5 to 21 ng/dL (0.17 to 0.73 nmol/L); labs vary. It runs in your browser and nothing you type leaves the device.
Each input has a specific job: - Total testosterone is the storage number. It counts free testosterone plus the albumin-bound and SHBG-bound fractions. - SHBG is the high-affinity binder. Testosterone stuck to SHBG is not available to tissues on a timescale that matters. - Albumin is the low-affinity binder. Testosterone attached to albumin is not free, but it releases quickly and is considered bioavailable in many tissues.
The first thing a free testosterone calculator tells you is whether the free fraction is low when the total is unremarkable. A man with total testosterone just below the bottom of the broad range can still have a normal free testosterone if SHBG is low. Another man with a normal total can have low or low-normal free testosterone if SHBG is high. That is why the calculator exists.
Why total testosterone alone misreads fatigue and low libido
Total testosterone is a weak screen for fatigue, low libido and slow recovery because it ignores binding. In adult men, the free fraction is roughly 1 to 2 percent of total testosterone. The rest is bound. If SHBG moves, the free fraction can change a lot while total testosterone stays nearly the same.
SHBG rises with age, high oestrogen, hyperthyroidism, liver disease and fasting or substantial weight loss. It falls with obesity, insulin resistance, high insulin, androgens and hypothyroidism. Albumin moves less often, but low albumin from chronic disease, kidney loss or malnutrition changes the calculation because the formula uses it.
The common mistake is to call a total testosterone of 420 ng/dL fine because it sits inside the broad laboratory range. Illustrative examples: if SHBG is 70 nmol/L, that same 420 ng/dL can leave very little free testosterone. If SHBG is 18 nmol/L, the same total can produce a normal free result. The broad range hides that difference.
The distribution in a typical adult male is shown as an illustrative split below.
The free 2 percent is the fraction that drives androgen receptor activity over minutes to hours. The albumin-bound fraction is a buffer that can replenish it.
Total testosterone is inventory. Free testosterone is what your tissues can actually use.
What the Vermeulen equation does with the numbers
The free testosterone calculator uses the Vermeulen 1999 method, the equation clinicians usually mean when they say calculated free testosterone. It models the binding equilibrium between testosterone, SHBG and albumin, using association constants for both binding proteins. It does not take a fixed percentage of total testosterone.
That matters because the relationship is not linear. SHBG has high affinity but saturable capacity. At high SHBG concentrations, more total testosterone is bound, and the free fraction falls faster than a fixed percentage would predict. At low SHBG, the free fraction rises. The equation accounts for that curve.
Two cases with the same total testosterone show why SHBG matters.
| Case | Total testosterone | SHBG | Free testosterone |
|---|---|---|---|
| High SHBG | 450 ng/dL | 60 nmol/L | 6.1 ng/dL (low-normal) |
| Low SHBG | 450 ng/dL | 20 nmol/L | 11.7 ng/dL (normal) |
Scroll the table sideways for the rest →
Illustrative: calculated with typical albumin values (4.3 g/dL) and varying SHBG. Free testosterone shown in ng/dL.
The table is illustrative, but the direction is what matters. In a symptomatic man, high SHBG with a normal total testosterone is not reassuring.
What if your lab did not measure SHBG?
Without SHBG there is no reliable calculated free testosterone. Calculated free testosterone shortcuts usually omit SHBG or assume a default albumin value; no reputable shortcut uses total testosterone and albumin alone. A Vermeulen free testosterone result without measured SHBG is unreliable. It is like estimating LDL from total cholesterol without triglycerides or HDL.
Direct free testosterone immunoassays can disagree with calculated free testosterone, so measured SHBG is preferred when the question is fatigue or low libido. The additional result is more useful than relying on a direct assay or a guess.
This is also where bioavailable testosterone gets confused with free testosterone. Bioavailable testosterone is free plus albumin-bound. Some labs report it, and it can be normal when free testosterone is not. Vermeulen's method gives free testosterone, not total bioavailable testosterone, so do not compare the two as if they are the same.
When the result is worth a medical discussion
A free testosterone result outside the relevant reference range, or a normal total testosterone with a low or low-normal calculated free testosterone, is a signal. It is not a diagnosis. Low free testosterone can come from high SHBG, true hypogonadism, chronic disease, severe underfeeding or a single lab variation.
The calculator returns a band for the relevant reference range, but children, pregnancy, oral contraceptive use, and testosterone therapy have different expected bands and draw timing. In those settings, the same equation exists but the reference ranges and timing of the draw matter even more, so treat the number as a data point for the clinician managing that situation.
If the pattern repeats on a second measurement, take both numbers to a doctor. The calculator is most useful as a way to bring the hidden number into view, not as a reason to self-treat.
See it in your own blood test
Upload a lab report at https://healthosx.com/blood and the result screen works out free testosterone when the report has total testosterone, SHBG and albumin. A second upload shows how each marker moved since the last one. Or open the free testosterone calculator and type the three numbers in now.
Written by
HealthOS Research