Free Androgen Index: how it is calculated and what it can do
The essentials at a glance
The Free Androgen Index is a calculated value derived from two laboratory measurements. The formula is: testosterone in nanomoles per liter multiplied by 100, divided by SHBG in nanomoles per liter (Urology Textbook, Manski D, accessed 13 August 2026). The result is a unitless number.
The index describes the proportion of testosterone that is not calculated to be tightly bound to the transport protein. The same source gives men an age-dependent range between 30 and 150 and classifies values below 30 as an indication of relative testosterone deficiency.
You will first read what the index was designed for. This is followed by the formula step by step, the documented limitation, a fact check of three common statements, a comparison with calculated free testosterone, the role in women, and the limitations of a calculated value.
What to expect in this article
1. What the index was designed for
2. The one limitation that puts everything into perspective
3. The formula step by step
4. Three statements about the index in the fact check
5. Why the unit determines the result
6. Index, free testosterone, and total value compared
7. How to interpret an index value
8. The index in women
9. Who benefits from the index
10. What a capillary blood test can show here
11. Limitations: what a calculated value cannot do
12. What matters for the Free Androgen Index
Frequently asked questions
Sources
What the index was designed for
Most testosterone in the blood is present in bound form. Sex hormone-binding globulin binds tightly, and the hormone’s strong binding to the transport protein prevents biological activity (Urology Textbook, Manski D, accessed 13 August 2026).
This creates a diagnostic problem. The total value includes the bound fraction, and two people with the same number may differ considerably in availability.
The Free Androgen Index is the simplest attempt to solve this problem. It relates the two quantities that can both be measured without a specialist laboratory and produces a number that says something about their distribution.
Why there are several methods at all
The most accurate method would be direct measurement of free testosterone. The MSD Manual, Professional Edition, notes that this measurement requires equilibrium dialysis, which is technically difficult and not available everywhere (as of April 2025).
This leaves two calculation methods. One includes albumin and provides a calculated free testosterone value. The other leaves albumin out and provides the index.
Both methods answer the same question with different levels of effort. Which one appears on a report depends on the laboratory and the profile requested.
Why a ratio is useful at all
A ratio has a practical advantage over two individual values. It answers the question that actually matters in a single number instead of two.
The total value alone does not provide this answer. The MSD Manual gives a normal range of 300 to 1,000 nanograms per deciliter for it (as of April 2025), and within this range of more than threefold variation, a single number says little about availability.
The SHBG value alone is just as uninformative. It describes binding capacity, and without the hormone to be bound, that capacity has no object.
Only together do the two provide meaningful information. The index does exactly that, accepting that the two underlying numbers are no longer visible in the result.
The one limitation that puts everything in perspective
Before introducing the formula, the most important qualification belongs at the beginning. It determines how much weight an index value can carry at all.
Documented source
“The free androgen index is considered less reliable than the calculated determination of free testosterone because the albumin concentration is not taken into account.”
Dr. med. Dirk Manski
Urologielehrbuch, section SHBG and Free Androgen Index, accessed 13 August 2026
The sentence identifies the price of simplicity. The index needs two values instead of three, and that is precisely the third value it lacks.
Albumin binds testosterone loosely, and this loosely bound fraction is available to the tissues. Omitting it means calculating based on a simplified assumption about distribution.
When reading this, it means: The index is a guide. If calculated free testosterone is listed on the same report, it is the more reliable of the two figures.
The formula step by step
The calculation is so simple that you can follow it mentally. It is: Free Androgen Index equals testosterone in nanomoles per liter times 100, divided by SHBG in nanomoles per liter (Urologielehrbuch, Manski D, accessed 13 August 2026).
It contains three components, each with a role. The testosterone in the numerator represents the amount present, SHBG in the denominator represents binding capacity, and the factor 100 puts the result into a range that is easy to read.
What the calculation means in practical terms
When a high amount of testosterone is set against low binding capacity, the fraction becomes large and the index high. When little testosterone is set against high binding capacity, it becomes small.
This leads to a feature that often surprises people in everyday practice. The index can fall without any change in testosterone—namely, when SHBG has increased.
And it can rise without more testosterone being produced if SHBG decreases. An index value alone therefore does not indicate which of the two quantities has changed.
This is not a flaw in the formula, but the way it is designed. A ratio describes a relationship, not the two numbers from which it is derived.
That is why all three numbers belong on the report
A result showing only the index gives away information. Seeing testosterone and SHBG alongside it allows you to ask about the cause yourself.
In practical terms, when reading results: write down all three numbers, even if you are initially interested in only one. With the next test result, that is precisely what determines whether a comparison is possible.
Two hypothetical cases with the same index
The following comparison is a calculation example and explicitly not a real-world case. Two men have the same Free Androgen Index, but the two paths leading to it are different.
In the first case, both testosterone and SHBG are in the middle range. The ratio is appropriate, and the two components give no reason for further questions.
In the second case, both values are low. The quotient produces the same number because the numerator and denominator have both shifted downward, yet the situation is different.
Low testosterone with low SHBG raises questions that the index does not ask. That is precisely why it does not replace the two original values but summarizes them.
That is the general property of ratios. They condense information, and information is lost in the process. Anyone reading the index should know which information is lost.
Three statements about the index: a fact check
These three statements come up as soon as the index appears on a test result. All three can be checked against the available evidence.
Checked against the available evidence
Commonly claimed
“The index is more accurate than the total value.”
Documented
It takes binding capacity into account, which the total value leaves out. At the same time, it is considered less reliable than the calculated determination of free testosterone because albumin is not taken into account (urology textbook, Manski D, accessed August 13, 2026).
Commonly claimed
“An index below 30 means testosterone deficiency.”
Documented
According to the same source, values below 30 indicate a relative testosterone deficiency; the reference range for men is age-dependent and lies between 30 and 150 (accessed August 13, 2026). Indicating is not the same as proving, and the reference range of the laboratory performing the analysis remains decisive.
Commonly claimed
“I can calculate the index myself from any test result.”
Documented
The formula requires both values to be in nanomoles per liter. If the testosterone result is given in nanograms per deciliter, it must be converted first; otherwise, the calculation produces a meaningless number.
The third point is the one that most often goes wrong. It has nothing to do with medicine and everything to do with units, which is why it gets its own chapter.
Why the unit determines the result
Testosterone is commonly reported in two units. The MSD Manual, professional edition, gives the normal range in both: 300 to 1,000 nanograms per deciliter, corresponding to 10.5 to 35 nanomoles per liter (as of April 2025).
SHBG, on the other hand, is usually reported in nanomoles per liter. The urology textbook gives a range of 15 to 60 nanomoles per liter, depending on age and laboratory (accessed August 13, 2026).
That explains the source of the error. Anyone who enters testosterone in nanograms per deciliter into the formula without checking divides two quantities that do not match and obtains an index in a completely incorrect order of magnitude.
The check takes only a glance. If the unit next to your testosterone value is ng/dL, convert it before doing the calculation. If it says nmol/L, it is suitable.
How to recognize an incorrect order of magnitude
An error in the units becomes apparent when you know the expected range of the result. For men, the urology textbook gives an age-dependent range between 30 and 150 (accessed August 13, 2026).
If your calculation comes out to several thousand, you most likely entered testosterone in nanograms per deciliter. If it comes out to a number below one, something is probably wrong with the denominator.
This plausibility check does not replace a careful calculation, but it catches the most common error. And it costs nothing except a second look at the report.
The more convenient option is still to have the laboratory calculate the index. Both values are already listed there in the same unit, so no conversion is needed.
Index, free testosterone, and total testosterone compared
Three approaches lead to the question of available testosterone. The table compares them according to the same four criteria.
| Criterion | Total testosterone | Free Androgen Index | Calculated free testosterone |
|---|---|---|---|
| Required values | One: testosterone itself | Two: testosterone and SHBG, both in nanomoles per liter | Three: testosterone, SHBG, and albumin |
| What it outputs | A concentration with a unit | A unitless ratio | A concentration with a unit |
| Where its weakness lies | Completely ignores binding and becomes less sensitive after age 50 | Does not take albumin into account and is therefore less reliable | It remains a calculation and depends on three input values |
| When it is useful | As an initial guide for younger men | When only testosterone and SHBG are available and a quick assessment is sufficient | When all three values are available and the question needs to be answered more precisely |
The final line clarifies the roles. The index is the quick method, calculated free testosterone is the more careful one, and both answer the same question.
How to interpret an index value
The first step is to compare the result with the laboratory’s reference range, not with a number found on the internet.
A ratio indicates that something has shifted. It does not say which of the two numbers has changed.
The Institute for Quality and Efficiency in Health Care states the basic rule as follows: The reference range shown on your own laboratory report is decisive, and reference values may vary from one laboratory to another (as of April 2, 2025).
The second step is to examine the individual components. A low index with high SHBG tells a different story than a low index with low testosterone.
The third step is the time of day. Because testosterone is in the numerator, its daily rhythm carries over to the index. The MSD Manual recommends collecting the sample in the morning before 10 a.m. (as of April 2025), and this recommendation applies to the calculated value just as it does to the underlying value.
What affects the denominator
Because SHBG is in the denominator, all influences on this transport protein also shift the index. The urology textbook identifies two directions for these influences.
Factors that can increase it include age, estrogens, testosterone deficiency, hyperthyroidism, liver disease, and numerous medications. Factors that can decrease it include androgens, hypothyroidism, metabolic syndrome, and steroid therapy (accessed 13 August 2026).
The list explains why an index value on its own carries little meaning. Two very different underlying conditions, such as hyperthyroidism and liver disease, can affect the denominator in the same direction.
For you as a reader, this means: An abnormal index is first a reason to examine its components, and then a reason to check the obvious explanations from this list. Both belong in a conversation, not in self-interpretation.
The index in women
The free androgen index is also determined in women, and the reference ranges differ considerably from those for men. This is to be expected because the underlying testosterone levels are several times lower.
The calculation remains the same, and so does the limitation. Albumin is missing here too, and here as well, the index is a guide, not a finding.
What is added is the menstrual cycle. Because hormone levels shift over the course of the cycle, interpretation depends on when the sample was taken. The sister article Hormone Levels and the Right Day of the Cycle explains what follows from this.
Who benefits from the index
The index is not a value everyone needs. The comparison makes that clear in both directions.
Useful for you if …
Your report lists testosterone and SHBG and you want a quick assessment of availability.
You want to compare two reports from the same laboratory and the same two values are available both times.
You want to understand why an unremarkable total value and noticeable symptoms can occur together.
You want to bring figures to a consultation instead of having them measured only at the first appointment.
Probably not if …
Your report already includes a calculated free testosterone value. In that case, it is the more reliable figure.
You want to derive a diagnosis from the index. A ratio describes a distribution, not a finding.
Your two values are given in different units and you do not convert them.
The sample was not collected in the morning. Testosterone’s daily rhythm carries through into the index.
What a capillary blood test can show here
A capillary blood self-test does not provide a diagnosis. For the index, it comes down to one question: whether testosterone and SHBG appear together on the same report.
The two tests from mybody®x (MYBODY Lab GmbH) differ in this respect. What they do and do not provide is shown on the cards.
Capillary blood test
HormonCheck | Men’s Hormone Test
Measures testosterone, SHBG, and albumin together and uses them to calculate free testosterone, along with estradiol, DHEA-S, LH, FSH, prolactin, and TSH. What the test does not provide: it does not report the Free Androgen Index as a separate value. Because all three input variables are available, the albumin-based calculation is more reliable anyway. The test does not provide a diagnosis.
Laboratory analysis 3–5 business days after sample receipt
Product page information, accessed 13 August 2026
Capillary blood test
Men’s Wellness Check | Men’s Health Test
Measures 17 values across several areas, including testosterone, cortisol, TSH, the cholesterol profile, homocysteine, liver and kidney values, and PSA. What the test does not provide: it does not measure SHBG. This means the denominator of the formula is missing, so a Free Androgen Index cannot be calculated from its report. It is not the right test for this question.
Laboratory analysis 3–5 business days after sample receipt
Product page information, accessed 13 August 2026
Four points determine whether an index value becomes useful information.
Check the units
Both values must be given in nanomoles per liter; otherwise the formula produces nonsense.
Measure in the morning
Testosterone’s daily rhythm carries through the numerator into the index.
Record both components
Without testosterone and SHBG, it will not be possible to tell later what has changed.
Discuss anything abnormal
A low index accompanied by symptoms should be taken, along with the report, to a medical practice.
Chapter at a glance
The Free Androgen Index is derived from testosterone and SHBG and does not require a separate test—only a report listing both values. If albumin is also available, calculated free testosterone is the more reliable figure. Four points are crucial: check the units, measure in the morning, record both components, and discuss anything abnormal.
Limitations: what a calculated measure cannot do
The first limitation is the documented restriction. The index does not account for albumin and is therefore considered less reliable than mathematically determined free testosterone (Urology textbook, Manski D, accessed 13 August 2026).
The second limitation is the ambiguity of the ratio. The same index value can result from very different combinations of testosterone and SHBG, and the number alone cannot distinguish between these cases.
The third limitation concerns the cause. A low index does not answer where something is functioning differently in the regulatory system. For that, the MSD Manual describes measuring total testosterone, FSH, and LH at the same time (as of April 2025).
The fourth limitation is comparability. Because reference ranges can vary between laboratories and the index is derived from two lab-specific measurements, two values from the same laboratory are easier to compare than two from different laboratories.
What the index says about symptoms
The symptoms that lead people to have hormone testing are usually nonspecific: fatigue, reduced drive, and decreased libido. They have many possible causes.
An index assesses one of these causes and does not rule out the others. Lack of sleep and insufficient recovery, for example, rank high on the list of causes of persistent fatigue (MSD Manual, professional edition, as of April 2025), and no hormone value changes that.
The reverse is also true. An unremarkable index does not mean that the symptoms are imagined. It means that one possible explanation appears unremarkable according to this calculation.
That is precisely where the honest benefit of a calculated measure lies. It answers one of several questions, and it does so quickly. You should not expect more from it.
What matters about the Free Androgen Index
If you take away one practical step from this article, let it be this: Write down the two numbers from which each index was calculated next to it.
The reason is unremarkable. A ratio cannot later be broken down into its components, and without those components, the next test result leaves it unclear which side has changed.
The starting question was what the index accomplishes. The most honest answer is: quick guidance with a known weakness. If you have all three values, use the longer calculation and get the better figure.
It is a fair objection that an article about a formula leaves little to say if the formula ultimately turns out to be the second-best solution. The benefit remains, nonetheless: anyone who knows how the index is calculated can immediately tell from their own results whether the better figure is listed alongside it or is missing.
And anyone who knows that SHBG is in the denominator will not hastily interpret a falling index as falling testosterone. That is the key insight from a formula with two components.
Frequently asked questions
How is the free androgen index calculated?
The formula is: testosterone in nanomoles per liter multiplied by 100, divided by SHBG in nanomoles per liter (Urology Textbook, Manski D, accessed 13 August 2026). The result is a ratio without a unit. Both values must be given in the same unit; testosterone in nanograms per deciliter must be converted first.
What is considered a normal value?
For men, the Urology Textbook gives an age-dependent range of 30 to 150 and classifies values below 30 as an indication of relative testosterone deficiency (accessed 13 August 2026). The decisive factor remains the reference range stated on your own laboratory report, because reference values can vary from one laboratory to another (IQWiG, as of 2 April 2025).
Is the index better than free testosterone?
No. The free androgen index is considered less reliable than the calculated determination of free testosterone because it does not take albumin concentration into account (Urology Textbook, accessed 13 August 2026). If both figures appear on a report, the calculated free testosterone is the more reliable one.
Why is my index decreasing even though my testosterone stayed the same?
Because SHBG is in the denominator. If this transport protein increases, the index decreases even if testosterone levels have not changed. One known reason is age: Since SHBG increases with age, total testosterone after the age of 50 is a less sensitive parameter (MSD Manual, professional edition, as of April 2025).
Can I calculate the index myself?
In principle, yes, provided both values are given in nanomoles per liter. The real difficulty is interpretation: An index without the two underlying figures, the time of sample collection, and the laboratory’s reference range remains a number without context. Abnormal results should be discussed with a doctor.
Next step
Three values instead of two
Anyone who has testosterone, SHBG, and albumin measured together does not need to take the detour via the index. The HormoneCheck measures all three and uses them to calculate free testosterone. It does not provide a diagnosis and does not replace medical evaluation.
Go to the HormoneCheck Go to the Men’s Wellness CheckRead more
You might also be interested in this
The denominator of the formula, explained in detail.
The numerator of the formula and the question behind it.
Sources
- Manski D: Urology Textbook, section on Sex Hormone-Binding Globulin (SHBG) and Free Androgen Index, accessed 13.08.2026 – urologielehrbuch.de
- MSD Manual, Professional Edition: Male Hypogonadism, Jimbo M (as of April 2025) – msdmanuals.com
- Institute for Quality and Efficiency in Health Care (IQWiG): Understanding Laboratory Values Correctly, as of 02.04.2025 – gesundheitsinformation.de
- MSD Manual, Professional Edition: Fatigue, Wasserman MR (as of April 2025) – msdmanuals.com
The formula for the Free Androgen Index, the verbatim quote regarding the lack of consideration of albumin, the reference range between 30 and 150 for men, the classification of values below 30, the SHBG reference range of 15 to 60 nanomoles per liter, and the statement about binding to the transport protein are taken from source [1]. The normal range for total testosterone in both units, the statement about equilibrium dialysis, the increase in SHBG with age, the recommendation to take the sample before 10 a.m., and the note on determining FSH and LH simultaneously are taken from [2]. The explanation of the reference range and the note about variations between laboratories are taken from [3]. The classification of sleep deprivation as a cause of persistent fatigue is based on [4]. Information on price, values, sample type, and laboratory comes from mybody®x product pages, accessed on 13.08.2026; processing times follow the central specification for blood tests. All sources were accessed and reviewed on 13.08.2026.
mybody®x Editorial & Specialist Team
Laboratory diagnostics Blood analysis interpretation Nutritional science Nutrigenetics
This article was created by the mybody®x editorial and specialist team. The team combines laboratory diagnostics, nutritional science, and the interpretation of blood analyses. Those who contribute to it are listed on the authors' page.
Published on 13.08.2026 · Last updated on 13.08.2026
The content is intended for general information and does not replace medical advice, diagnosis, or treatment. Reference ranges depend on the laboratory, method, and age—always follow the information on your test report.
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