By Mike Kocsis | 10 minutes read | Last updated: September 29, 2025 Categories: Testosterone
Medically Reviewed by Dr. George Touliatos
Free testosterone, also called biologically active testosterone, is 2% to 5% of your total testosterone that produces an end-effect on organs and tissues. The rest of the testosterone in your bloodstream is bound to proteins and is inactive.
If you suspect having abnormal free testosterone levels, talk to your doctor and have a testosterone blood test.
Keep reading the article to learn more about free testosterone, including its normal range in men, functions, and more.
What does free testosterone do?
Free testosterone is the active version of testosterone that’s involved in various processes in men. Here are the common functions of free testosterone:
- During puberty, the main role of free testosterone in males is to develop secondary sex characteristics like the formation of body hair, growth of facial hair, deepening of the voice, enlargement of the larynx, increase in muscle mass, etc.
- It controls the growth of testicles and increases sex drive.
- It is involved in the development of muscle mass and strength. It increases the production of growth hormones which further increase muscle mass.
- It boosts total bone mass. That is why people with testosterone deficiency are prone to bone fractures.
- It metabolises body fat and helps people burn excessive fat cells fast.
- Testosterone also encourages bone marrow to produce more healthy red blood cells.
- Testosterone is also known to control some central behaviours like dominance and aggression. It increases confidence level and sense of competitiveness.
What are normal free testosterone levels by age in men?
The table below shows the optimal total testosterone, free testosterone, and sex hormone binding globulin (SHBG) bound testosterone levels in men.
| Age | Total Testosterone (nmol/L) | Free Testosterone (nmol/L) | SHBG Bound Testosterone (nmol/L) |
| 25-34 | 21.4 | 0.43 | 35.5 |
| 35-44 | 23.1 | 0.36 | 40.1 |
| 45-54 | 21 | 0.31 | 44.6 |
| 55-64 | 19.5 | 0.29 | 45.5 |
| 65-74 | 18.2 | 0.24 | 48.7 |
| 75-83 | 16.3 | 0.21 | 51.0 |
| 85-100 | 13 | 0.19 | 65.9 |
Free vs total testosterone
Around 95% to 98% of the testosterone in your body is attached to either albumin or sex hormone-binding globulin (SHBG) proteins; thus, it is called bound testosterone.
The rest of the 2% to 5% of the testosterone circulating in the unbound form freely in the bloodstream is called free testosterone.
The sum of both free and bound testosterone makes the total testosterone.
Free testosterone can directly reach a site and attach to the cells without getting the help of any transport protein.
On the contrary, bound testosterone needs the assistance of transport proteins like SHBG to reach the target site and enter the cells to perform its function.
That is why a high concentration of SHBG in the blood may lead to low free testosterone levels.
Free vs bioavailable testosterone
As mentioned earlier, free testosterone is the form of male sex hormone that’s not bound to any carrier protein in the bloodstream.
In contrast, bioavailable testosterone is the sum of free testosterone and albumin bound testosterone that can quickly become active.
All the testosterone is produced in testicles and adrenal glands after getting signals from the pituitary gland. Most of it attaches to SHBH and albumin proteins as it enters the bloodstream.
What are the signs of low free testosterone?
If your testosterone levels are lower than normal, then you may experience the following symptoms.
Low testosterone signs in men:
- Loss of body hair
- Fatigue, mood changes, and lack of concentration
- Low sex drive
- Trouble in maintaining and attaining an erection
- Decreased semen volume
- An increase in the total body fat
- Reduction in the total bone mass
- Small testicles
Low testosterone signs in women:
- Muscle weakness and reduced muscle mass
- Bone weakness
- Fatigue and low energy levels
- Trouble falling asleep
- Increase in the bodyweight
- Low libido and sexual function
- Fertility issues
- Vaginal dryness
If you are experiencing such symptoms, have your hormonal status evaluated and talk to your doctor to identify what is causing these issues.
How to measure free testosterone levels?
There are two important testosterone tests that are used to measure free testosterone concentration.
One is called the total testosterone test. This test measures the amount of total testosterone as well as the amount of free testosterone. The other test is called the free testosterone test, and it measures the concentration of only free testosterone.
These tests are simple blood tests where a blood sample is drawn from your vein and stored in a tube that is sent to the laboratory for testing.
Testosterone tests are performed in the morning because you have the highest testosterone levels at that time.
If your test results show that you have low or high testosterone levels, then another testosterone test might be performed for accurate measurements and confirmation. The need to repeat the test occurs because testosterone levels can fluctuate from day to day due to multiple reasons.
Further testing, like testing of the luteinizing hormone and follicle stimulating hormone may be done to confirm cause of this hormonal issue.
Why should you test free testosterone levels?
Suppose you are experiencing low testosterone symptoms, so you decide to have a total testosterone test. When the test results come out, you discover that your total testosterone levels are normal. Still, you experience low testosterone symptoms – so what can be causing this issue?
There is a possibility that these symptoms are due to the low levels of free testosterone. That’s why, in addition to testing total testosterone, you should also get your free testosterone levels checked.
Can you have normal total testosterone and low free testosterone?
Yes, men can have normal total testosterone but still low free T levels.
Studies show that total testosterone deficiency is not the only cause behind low T symptoms. You can experience these signs even if your total testosterone is normal but free testosterone is low. Therefore, it is important to have your free testosterone levels checked too.
Sometimes, health care providers may only check the total testosterone level and ignore the levels of free testosterone. This can lead to misdiagnosis.
If you experience low overall testosterone production and hypogonadal signs, talk to your doctor about total and free testosterone tests to get his suggestions for your specific case.
How to increase free testosterone?
Normally, it is first suggested to try natural means to boost your testosterone levels. The following are some of the best healthy lifestyle choices that can help increase the active form of testosterone.
- Regular exercise can help you elevate testosterone levels. High-intensity and weight-lifting exercises are considered more effective.
- Eating a balanced diet consisting of the right amount of three essential nutrients (carbs, protein, and healthy fats) may also boost testosterone.
- Try to reduce regular stress levels.
- Take a sunbath to increase your vitamin D3 levels which may help increase testosterone.
- Maintain a healthy lifestyle and get at least eight hours of sleep every day.
- Reduce the intake of alcohol.
- Eat testosterone boosting foods like beef, tuna, egg yolk, low-fat milk, beans, etc.
When natural means fail to increase low free testosterone levels, your doctor may prescribe a hormone therapy like testosterone replacement therapy (TRT).
Does TRT increase free testosterone?
Yes, testosterone therapy can help men increase their free testosterone levels.
TRT is prescribed to men with hypogonadism or abnormally low testosterone levels. In this treatment, testosterone injections, pellets, gels, creams, patches, etc., are given to the patients to increase their hormone levels.
Even though it is an effective treatment, it also has some side effects. Therefore, it is not suitable for all men, especially for those with medical conditions like a prostate cancer history, uncontrolled sleep apnea, etc.
Talk to your doctor about the good and bad effects of TRT to decide whether you are the right TRT candidate.
Free testosterone calculator
Use our free testosterone calculator to measure its value. For this, you need to know total testosterone, albumin and SHBG levels in your serum.
Free testosterone test in the UK
If you’ve come here and simply want to order a free testosterone test and are based in the UK, then follow this link for your free testosterone test to purchase our ‘testosterone plus test’ which will give you a full breakdown of your total testosterone.
Summing it up
Free testosterone is the testosterone that moves freely in your bloodstream (with no bound proteins) and can easily get absorbed in the cells to perform its functions.
Low free and total testosterone levels in the serum can cause symptoms like fatigue, low sex drive, erectile dysfunction, low bone mass, etc. If low T is disturbing your life, get a suitable treatment for healthy testosterone levels on time.
References
Grossmann, M., Jayasena, C.N. and Anawalt, B.D., 2023. Approach to the patient: the evaluation and management of men≥ 50 years with low serum testosterone concentration. The Journal of Clinical Endocrinology & Metabolism, 108(9), pp.e871-e884.
Al-Zoubi, R.M., Yassin, A.A., Alwani, M., Al-Qudimat, A., Aboumarzouk, O.M., Zarour, A. and Al Ansari, A., 2021. A systematic review on the latest developments in testosterone therapy: Innovations, advances, and paradigm shifts. Arab journal of urology, 19(3), pp.370-375.
Lolck, K.V., Alcazar, J., Kamper, R.S., Haddock, B., Hovind, P., Dela, F. and Suetta, C., 2025. Compared to total serum testosterone, calculated free testosterone has a stronger association with lean mass, muscle strength, power, and physical function in older men. Aging clinical and experimental research, 37(1), pp.1-11.
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About the Author: Mike Kocsis
Mike Kocsis has an MBA with a focus on healthcare administration and is an entrepreneur and medical case manager for Balance My Hormones which offers medical services in the UK and Europe. Mike has over 20 years of experience in the healthcare sector, much of that working with people who have hormone imbalances. Mike has appeared on podcasts and radio and is an expert speaker on the subject of hormone imbalance. He specialises in Testosterone Replacement Therapy and Hormone Replacement Therapy (HRT) and has helped thousands of people suffering from hormone imbalances recover and regain control of their lives. You can follow him on LinkedIn and on the Balance My Hormones YouTube Channel.
Last update: September 29th, 2025
