T Nation

Measurement of Free Testosterone

FreeT is often measured in clinical practice and these values are frequently discussed; here is some background info and literature references

FreeT can be measured/estimated by three different principles:

  • equilibrium dialysis with subsequent detection by ECLIA or LC/MS -> gold standard but frequently not available because the test can not be automated
  • ‚direct‘ measured using a T analogue -> used widely because the test can be automated
  • estimation of freeT using total T and SHBG levels via eg the Vermeulen equation

Unfortunately the most widely used test, the ‚direct‘ measurement using a T analogue is worthless as it measured a constant fraction of total T, but not freeT.

If you want to measure/estimate freeT than either use the more expensive equilibrium dialysis method or estimate freeT via the Vermeulen equation (calc freeT). There is a strong correlation between freeT levels as measured by the equilibrium dialysis method and the calculated freeT using the Vermeulen equation.

From Fiers et al, 2018

From Swerdloff, 2008 regarding the direct analog based freeT test:

‚ Should the analog-based free t assay be used in clinical practice?

Because the analog-based assay does not measure free T, it makes no sense to label it as such. Because it tracks total T but has not been shown to add to the values of total T results, we do not recommend its use.‘


Quest Labs offers the dialysis method. You can find this method grouped with a TT test for under $50 online if your state allows you to order your own blood work.

My doc also mentioned that calculated T is a much better method than direct free T, but I never found a source to back that up. Someone on the forums a while ago had a big conflict between their direct free T and calculated free T, I assumed it was because of what you described, but didn’t say anything because I wasn’t sure about it. So thank you for this.

To add to the controversy, theres the “old” calculation and the new TruT method, supposedly FDA approved. It is claimed one has roughly double the levels of the other.

I have read that not all calculators are accurate either, only the Tru-T calculator should be used according to my research.

Why TruT™

Prevailing calculators are inaccurate

It has been demonstrated that the calculated free testosterone derived using commonly available online calculators, which are based on simpler models assuming simple 1:1 binding of testosterone and SHBG [1,2], deviate from measurements obtained by equilibrium dialysis by as much as 40% [3-9].

The Endocrine Society position

Current methods for measuring free testosterone (fT) are technically challenging and not accurate. The widely used direct immunoassay and tracer analog techniques for measuring fT have been shown to be inaccurate. Equilibrium dialysis, the reference method against which other methods are compared, is labor-intensive and cumbersome, and therefore has had limited clinical adoption. Recently, Endocrine Society’s Expert Panel acknowledged the experimental problems in fT measurements and concluded that “…the calculation of free testosterone is the most useful estimate of free testosterone in plasma…” For this reason they advocate for indirect “calculator” based methods, where free testosterone can be computed from the total testosterone, SHBG, and albumin concentrations.

However, we have demonstrated that even the calculated fT values derived from the prevailing equations, based on linear law-of-mass action models or empiric equations, differ systematically from free testosterone measured by equilibrium dialysis by as much as 40%.

The http://www.issam.ch/freetesto.htm is not accurate.

I ran some test numbers. TT 1500, SHBG 50. TruT gives a free T of 52 and the more traditional issam.ch gives a free T of 32.

Using a SHBG of 20 (vs 50 above) in the TruT gives almost the same result! Free T increases by only 4 points.

@systemlord, all of the people feeling brilliant with free T of 30-60 are using the old calculators. All the good docs @dbossa posts about also use the old formula. My opinion is the TruT is rubbish. The “research” you have posted is a cut and paste job of the TruT websites self promotion material. Hardly credible. Forgetting the fact 20 vs 50 SHBG hardly changes the numbers, since you claim TruT is the only one to be used, in my books thats a good enough reason to completely ditch it.

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The TruT calculator is based on the formula developed by Zakharov et al.
A direct comparison of measured freeT using the equilibrium dialysis method and calculated freeT levels (Vermeulen, Zakharov and Ly) was done by Fiers et al (linked and graphically displayed in the post above).
The data demonstrated good agreement between measured T and calculated T according to Vermeulen. Calc freeT according to Zakharov systematically overestimated freeT, especially in the higher SHBG range. So according to this comparison the Vermeulen equation is more ‘accurate’.

However, the jury is still out whether the equilibrium dialysis method really accurately quantifies freeT. In the end its about linking freeT levels to clinical symptoms of androgen deficiency. At least according to Zakharov et al, they claim that their calc freeT was superior in predicting clinical symptoms in the European Male Aging Study.

Anyhow, i personally would go with the Vermeulen equation as there is simply more clinical experience available using this formula. Tru T using the Zakharov equation appears to overestimate freeT.

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Through research here is the formula for free t.

Free t is in range = hard dick, being able to maintain muscle, morning wood, did I say hard dick

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You bring up a really good point. All labs, regardless of test methods, need to be interpreted within the range of normal values for that particular laboratory and test method. Comparing raw numbers for Free-T across labs and test methods is an apples to oranges comparison.

The one caveat in interpreting the Free-T results based on normal values is that you have to throw away the ‘age-adjusted’ ranges and interpret the results with the ranges set for a young male age group. SHBG increases with age and T production decreases with age, the net result are lower Free-T normal ranges for older guys (like me). While being ‘normal’, it does not imply optimal. I prefer to have my hormones optimized to what is normal for a much younger age group.

I also can corroborate the latter part of your statement when I optimize my Free-T with the direct method for a 20-30 year old guy. My body simply functions better (by whatever method one uses for that assessment).

Also as we age our cells become less efficient and add in all the the toxins in the environment blocking hormones in various ways and normal isn’t good enough anymore for some people. Just take @dbossa as an example, he is symptomatic at the top of the reference ranges.

You’ve also got AR sensitivity. density etc… Those with PAIS will be symptomatic at 3x the top of the ref ranges… also depends on individual set points, look at @aaronca, his TT prior to using AAS (with high FT to boot, untreated to SHBG) was 1100, he doesn’t feel too great at 500… understandably

It’s “FDA Registered”… Weasel words to make you think it’s something credible.


I personally believe that morning wood is a symptom of adequate Free T. Libido and ability to maintain erection during sex, not as much, however I am sure there is a correlation there.

I’ve had morning wood and good enough libido when my TT was 279 and FT god knows what it was.
Now that my TT is 800 and FT is 24.9 ng/dL (according to the FT calculator) i’ve only slightly better libido and still maintain morning wood.

On my 9th month on TRT i think i’ve come to a conclusion that my low T was a result of low cortisol and hypothyroidism.

@johann77 isnt it best to calculate it based on all that complexity?

I think this is what they used in my country. And the unit they give it in does not make any sense - ng/l. For example last time I measured it it was 17ng/l, the range was 8-22, looking good, right?
But when I calculated it from SHBG and albumin it was 8ng/dl, which makes much more sense.
Can you elaborate on that please?

Makes sense. It always spits out results about double what I get elsewhere so I haven’t put much faith in it.

Yes this new calculator is a total bullshit and on excel male praise it like a gospel

Why people cannot understand this numbers are relative in the context to only know where most people feel optimal?

Yeah I saw that too and thought my levels were crazy high initially, but using the old free T calculator it matches my blood tests almost exactly so I think that’s more likely to be correct in my case.

The “all that matters is how you feel” argument makes it super subjective and comes off as an excuse to take however much you want to some people. Right or wrong, it does. People like their numbers.

Yes, but for 10 years the progressive doctors have found some optimal ranges on the old free t calculator, and now FDA tries to persuades us we have to lower our dose…that is crap. Otherwise I agree with you