Free Testosterone Calculator

Q: How much of your testosterone is free?

Only about 1% to 4% is free. 2 The rest is bound: About 33% to 54% is held loosely by albumin. 2 Most of the rest is stuck tightly to SHBG. So out of a total testosterone of 500 ng/dL, only about 5 to 20 ng/dL is free.

Q: Why is my total testosterone normal but my free testosterone low?

High SHBG can do this. 3 SHBG locks up testosterone, so a big share of your total is unusable. Your total number looks fine, but very little is free. Things that raise SHBG include aging, advanced liver disease, an overactive thyroid, high estrogen, anticonvulsants, and extreme weight loss. 2

Q: How do you convert free testosterone from pg/mL to pmol/L?

Multiply pg/mL by 3.467 to get pmol/L. To go the other way, divide pmol/L by 3.467. Example: 100 pg/mL × 3.467 = 347 pmol/L. The number comes from testosterone's molecular weight of 288.42 g/mol.

Q: How do you convert total testosterone from ng/dL to nmol/L?

Multiply ng/dL by 0.0347 . To go back, multiply nmol/L by 28.84. Example: 500 ng/dL × 0.0347 = 17.35 nmol/L.

Introduction

Most of the testosterone in your blood is stuck to proteins. Much of it sticks tightly to a protein called SHBG (sex hormone-binding globulin), and 33% to 54% sticks loosely to albumin.² Only 1% to 4% floats free.² That free part is what your body can actually use.

A normal blood test usually shows total testosterone, which adds all of it together. That number can look fine even when your free testosterone is low. This can happen when SHBG is high, such as with aging, an overactive thyroid, or advanced liver disease.²

This free testosterone calculator works out your free and bioavailable testosterone from three lab values: total testosterone, SHBG, and albumin. It uses the published formulas from Vermeulen and Södergård, and shows them side by side so you can compare them.

You get your free testosterone in pg/mL, pmol/L, and ng/dL, plus your bioavailable testosterone and the percent that is free. You also get a full step-by-step math breakdown, a chart of how your testosterone is split between free and bound forms, and the same results from each formula side by side.

This tool is for learning and is not a diagnosis. Talk to your doctor about what your results mean for you.

How to use our Free Testosterone Calculator

Enter your lab results and pick a formula. The calculator shows your free testosterone, your bioavailable testosterone, and the percent of testosterone that is free.

Calculation Formula: Choose the method you want. Vermeulen (1999) is the most widely used.² Södergård (1982) uses slightly different binding constants.

Total Testosterone: Type the total testosterone value from your blood test. Then pick the unit your lab used: ng/dL, ng/mL, nmol/L, nmol/dL, or nmol/mL.

SHBG: Type your sex hormone-binding globulin result and pick nmol/L or µg/mL. SHBG holds on to testosterone, so a high level lowers your free testosterone.

Albumin: Type your albumin result and pick g/dL or g/L. If you do not have this test, leave the default value of 4.3 g/dL.

Calculate and Reset: Results update as you type, but you can press Calculate to run it again. Press Reset to go back to the default values.

What Is Free Testosterone?

Testosterone travels in your blood in three forms. Much of it is stuck to a protein called SHBG (sex hormone-binding globulin).⁴ That part is locked up and your body cannot use it. Some is loosely held by albumin, another blood protein. And a small part, about 1% to 4%, floats on its own.² That loose part is called free testosterone.

Free testosterone is the form that can enter your cells and do work. It helps keep muscle and bone strong, and it is needed for sexual function and for making sperm.³

Free vs. Bioavailable vs. Total Testosterone

Total testosterone: everything in your blood, bound and unbound.
Free testosterone: the unbound part your body can use right away.
Bioavailable testosterone: free testosterone plus the albumin-bound part.² Albumin holds on weakly, so this hormone can still be used.

Why SHBG and Albumin Matter

Your total testosterone can look normal while your free testosterone is low. This happens when SHBG is high, because more hormone gets locked away. SHBG often rises with age, advanced liver disease, an overactive thyroid, and some medicines.² SHBG drops with obesity, type 2 diabetes, and insulin resistance, which can make total testosterone look low even when free testosterone is fine.² That is why, when total testosterone is near the low end of normal or a condition changes SHBG, doctors also check free testosterone, measured by equilibrium dialysis or worked out from total testosterone, SHBG, and albumin.³

How Free Testosterone Is Estimated

Measuring free testosterone directly is hard and costly. Instead, labs and doctors use math formulas based on how hormones bind to proteins.² The most widely used is the Vermeulen formula from 1999.² The Södergård formula uses slightly different binding numbers, so the two formulas give slightly different answers, and it helps to compare them.²

Typical Levels

Normal ranges change with age, sex, and the lab doing the test, so compare your result with the range printed on your own lab report. In a large European study of aging men, sexual symptoms were linked with free testosterone below 220 pmol/L together with total testosterone below 11 nmol/L.²

Signs of Low Free Testosterone

In men, low levels are linked with tiredness, low sex drive, loss of muscle, more body fat, poor sleep, low mood, and trouble focusing, as well as erection problems and fertility trouble.³ In women, the test is most often done to look for high testosterone, which can cause acne, extra body hair, and missed periods.⁴

Getting an Accurate Test

The Endocrine Society recommends testing fasting morning total testosterone with an accurate assay, and repeating the morning test to confirm a low result.³ Because levels move day to day, one low result is not a diagnosis.

This information is for learning only. Talk to a doctor about your lab results and any treatment.

Formulas used

Albumin molar concentration

C_{alb} = \frac{Alb_{g/dL} \times 10}{66500}

Albumin binding factor N

N = 1 + K_{alb} \cdot C_{alb}

Quadratic coefficients for free testosterone

a = N \cdot K_{SHBG}, \quad b = N + K_{SHBG}(S_{eff} - T), \quad c = -T \quad \text{from } T = N \cdot FT + \frac{K_{SHBG}\, FT\, S_{eff}}{1 + K_{SHBG}\, FT}

Free testosterone (mass-action quadratic solution)

FT = \frac{-b + \sqrt{b^2 + 4aT}}{2a}

Effective SHBG binding capacity (one binding site)

S_{eff} = sites \times SHBG \times 10^{-9}

Bioavailable testosterone

BioT = N \times FT

Unit conversion of free testosterone

FT_{pg/mL} = FT_{nmol/L} \times 288.42, \quad FT_{pmol/L} = FT_{nmol/L} \times 1000

Percentage free testosterone

\%FT = \frac{FT_{nmol/L}}{T_{total,\ nmol/L}} \times 100

Frequently asked questions

What is a normal free testosterone level for a man?

It depends on age and on the lab. Testosterone production falls with age, and more than 20% of healthy men over 60 have levels below the range for young men.⁵

Every lab sets its own range, so compare your number to the range printed on your own report.

What is a low free testosterone level?

There is no single cutoff. In a large European study of aging men, sexual symptoms were linked with free testosterone below 220 pmol/L together with total testosterone below 11 nmol/L.² That is about 63 pg/mL of free testosterone.

A single low number is not a diagnosis. The Endocrine Society recommends confirming a low result by repeating the fasting morning test.³

How do you calculate free testosterone from total testosterone and SHBG?

You cannot use simple division. Free testosterone is found with a mass-action equation that models how testosterone binds to SHBG and albumin.² The steps are:

Change total testosterone and SHBG to nmol/L and albumin to mol/L.
Work out the albumin factor: N = 1 + (K_alb × albumin).
Solve a quadratic equation using the SHBG binding constant.

The answer comes out in mol/L. Convert it to nmol/L, then multiply by 288.42 to give pg/mL.

What is the Vermeulen formula for free testosterone?

It is a 1999 method by Vermeulen, Verdonck, and Kaufman that calculates free testosterone from total testosterone and SHBG.¹ It uses association constants of 1 × 10⁹ L/mol for SHBG and 3.6 × 10⁴ L/mol for albumin.²

It is the most widely used formula in research and commercial labs.² In its authors' study, calculated values were almost identical to values measured by equilibrium dialysis, except in pregnancy.¹ Later studies found larger differences between the two.²

How much of your testosterone is free?

Only about 1% to 4% is free.² The rest is bound:

About 33% to 54% is held loosely by albumin.²
Most of the rest is stuck tightly to SHBG.

So out of a total testosterone of 500 ng/dL, only about 5 to 20 ng/dL is free.

What is bioavailable testosterone?

Bioavailable testosterone is your free testosterone plus the part stuck to albumin.² Albumin holds on weakly, so that hormone can break loose and enter tissue.

Free and bioavailable testosterone are both seen as the active forms, while the part bound to SHBG is generally treated as unavailable.²

Why is my total testosterone normal but my free testosterone low?

High SHBG can do this.³ SHBG locks up testosterone, so a big share of your total is unusable. Your total number looks fine, but very little is free.

Things that raise SHBG include aging, advanced liver disease, an overactive thyroid, high estrogen, anticonvulsants, and extreme weight loss.²

What causes low SHBG?

Common causes are obesity, diabetes and insulin resistance, an underactive thyroid, polycystic ovary syndrome, nephrotic syndrome, acromegaly, and medicines such as glucocorticoids and androgens.²

When SHBG is low, your total testosterone can read low while free testosterone stays normal. That is why testing SHBG alongside total testosterone matters.

How do you convert free testosterone from pg/mL to pmol/L?

Multiply pg/mL by 3.467 to get pmol/L. To go the other way, divide pmol/L by 3.467.

Example: 100 pg/mL × 3.467 = 347 pmol/L.

The number comes from testosterone's molecular weight of 288.42 g/mol.

How do you convert total testosterone from ng/dL to nmol/L?

Multiply ng/dL by 0.0347. To go back, multiply nmol/L by 28.84.

Example: 500 ng/dL × 0.0347 = 17.35 nmol/L.

Do I need an albumin test to calculate free testosterone?

It helps, but it is not critical. Albumin changes free testosterone only a little compared to SHBG. This calculator fills in 4.3 g/dL. Leave that value if you do not have an albumin result.

Use your real albumin if you have liver disease, kidney disease, or malnutrition, since those conditions can drop it well below normal and change the result.

When is the best time to get a testosterone blood test?

In the morning, fasting. The Endocrine Society recommends measuring fasting morning total testosterone and repeating the morning test to confirm a low result.³

Do not test while you are sick or getting over an illness.³

Why do different formulas give different free testosterone results?

Each formula uses different binding constants and assumptions. Vermeulen's uses 3.6 × 10⁴ L/mol for albumin and 1 × 10⁹ L/mol for SHBG, while Södergård's uses 4.06 × 10⁴ and 5.97 × 10⁸.²

Stick with one method when tracking changes over time.

Is calculated free testosterone as good as a direct lab test?

The Endocrine Society accepts either equilibrium dialysis or an accurate formula for free testosterone.³ Calculated values cost far less to get.

The analog free testosterone immunoassay that some labs run is not a reliable measure.¹ Ask for total testosterone plus SHBG and albumin instead.

What are the symptoms of low free testosterone?

Signs in men include low sex drive, weak erections, tiredness, loss of muscle, more body fat, low mood, poor focus, and fewer morning erections.³

Many of these signs are not specific to low testosterone, so doctors also look for other health problems. A diagnosis needs both symptoms and clearly and consistently low blood levels.³

Does free testosterone drop with age?

Yes. Testosterone production declines steadily with age in men, and more than 20% of healthy men over 60 have levels below the range for young men.⁵ SHBG also rises with age, which locks more hormone away.²

What is a normal free testosterone level for a woman?

Each lab sets its own range for free testosterone in women, so compare your result with your report. For total testosterone, MedlinePlus lists 8.4 to 48.1 ng/dL for women aged 17 to 50 and 2.9 to 40.1 ng/dL over 50.⁴

Doctors use total and free testosterone to check for high androgen levels when diagnosing PCOS, the most common hormone condition in women of reproductive age.⁶ High testosterone can cause acne, extra facial or body hair, hair thinning, and irregular periods.⁴

How can I raise my free testosterone naturally?

Some causes of low testosterone may be reversible when a doctor treats the cause or stops the medicine behind it.³ These include:

Severe obesity. Losing weight can help.
Some sleep disorders, such as obstructive sleep apnea.
Alcohol and marijuana abuse.
Opioids and some other medicines. Do not stop a prescribed medicine without talking to your doctor.

See a doctor if levels stay low.

Can medications affect free testosterone results?

Yes. Opioids and high-dose glucocorticoid therapy can lower testosterone.³ Prednisone is a glucocorticoid. Oral contraceptives, anticonvulsants, and selective estrogen receptor modulators raise SHBG, which cuts free testosterone.²

Biotin supplements can also mess up some hormone immunoassays.³ For the LH and FSH tests that doctors use to find the cause of low testosterone, the Endocrine Society advises stopping biotin supplements for at least 72 hours before testing.³ Tell your doctor every drug and supplement you take, biotin included, before testing.

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