Free Testosterone: What It Is and Why It Matters

You probably know that testosterone is important for men’s health, but did you know there’s more than one type? Free testosterone is the active form your body can easily use because it’s not bound to proteins and can bind directly to androgen receptors.

On the other hand, testosterone bound to proteins makes up most of the testosterone in your bloodstream. But it can’t do much until it’s freed from the protein it’s bound to.  

In this article, we explore everything you need to know about free testosterone including what it is, how it works, and why it’s important for keeping you healthy.

Testosterone is the main androgen (AKA male sex hormone) and free testosterone is the form your body can use right away. It binds to androgen receptors, influencing many aspects of your health.

When reviewing bloodwork, doctors sometimes look at your “free testosterone fraction.” This measures the amount of bioavailable testosterone — testosterone your body can easily use — compared to the amount that’s attached to proteins.

Bioavailable testosterone includes both free testosterone and albumin-bound testosterone. Since it’s only weakly attached to the protein albumin, it’s still accessible to your body. That’s why albumin-bound testosterone is included in the calculation of bioavailable testosterone, even though it's not technically "free”.

Is one form of testosterone better than the other? How does your body use each type?

We go over the differences below.

Bound Testosterone

Most of the testosterone circulating in your blood — about 98 percent — is bound to proteins made by your liver called sex hormone-binding globulin (SHBG) and albumin. The remaining 2 percent is free or unbound.

The bond between testosterone and albumin is weaker than its bond with SHBG and it separates from the protein easily. This makes albumin-bound testosterone especially important for stabilizing fluctuating levels of testosterone in your bloodstream.

Free Testosterone 

Free testosterone isn’t attached to any proteins, so it can circulate in your blood and directly bind to receptors, where it exerts its effects. It plays a vital role in keeping your body functioning and influences things like

• Libido. Free testosterone is critical for sexual desire and performance in both men and women. 

• Muscle mass and strength. Total and free testosterone supports muscle growth and strength by helping your body create muscle tissue.

• Bone health. Healthy testosterone levels maintain bone density, reducing the risk of fractures and osteoporosis, especially as you age. 

• Mood and cognitive function. Research links low total and low free testosterone to depression, mood swings, and even cognitive decline in older men. 

• Fat distribution. Testosterone helps regulate how your body distributes fat. Low levels of total and free testosterone may mean more body fat.

• Energy levels. Feeling sluggish? Testosterone can influence energy levels. Some men with low testosterone levels report experiencing energy boosts with testosterone replacement therapy.

When you get a blood test for testosterone, it might measure either total testosterone, free testosterone or both. A free testosterone test looks at the unbound testosterone in your blood, while a total testosterone test measures both bound and unbound testosterone.

Your healthcare provider may check your total testosterone levels to help understand symptoms like low energy, erectile dysfunction, or a low mood, or track how well testosterone replacement therapy (TRT) is working.

In most cases, a free testosterone test isn’t necessary unless your healthcare provider needs a clearer picture on a suspected issue, especially when:

• Your total testosterone is low

• You have symptoms of low testosterone but your total levels are in the normal range

• There may be an issue with a protein called SHBG (sex hormone-binding globulin), which affects how much testosterone is usable

When it comes to testosterone, more isn’t necessarily better. What really matters is keeping it within a healthy range.

Typically, free testosterone makes up just 1-2 percent of the total testosterone in your bloodstream.

In the early morning, normal total testosterone levels fall between 300 and 1000 ng/dL. But the exact reference range can vary slightly depending on the lab. Some labs report serum testosterone levels in nanomoles per liter (nmol/L), with a typical range of  10–35 nmol/L.

The American Urological Association defines low testosterone as a total testosterone level of less than 300 ng/dL. 

Free testosterone levels are lower than total testosterone because only a small amount is unbound and active in the body. Normal ranges can vary depending on the lab, the testing method, and the person being tested. In general, free testosterone levels usually fall between about 6.8 and 26 picograms per milliliter (pg/mL).

Low Free Testosterone

Low total and free testosterone can take a toll on your quality of life. When levels dip too low — a condition called hypogonadism — you might notice changes like:

• Decreased libido and sexual function

• Trouble getting or maintaining erections (AKA erectile dysfunction)

• Fatigue and lack of energy

• Loss of muscle mass and strength

• Increased body fat

• Depression, mood swings, and irritability

• Difficulty concentrating and memory problems

Learn more about the signs of low testosterone. 

Your age, health conditions, lifestyle choices, and even certain medications can all influence your total and free testosterone levels. In general, when sex hormone-binding globulin (SHBG) levels go up, free testosterone usually goes down. SHBG attaches to testosterone in your blood, which stops your body from using it. So, more SHBG means less free testosterone is available to do its job.

Let’s break down each factor in more detail. 

Age

Testosterone levels peak in your teens and early 20s, then start to decline after 35.

Some research suggests free testosterone may drop faster than total testosterone as you age.

Health Conditions

Your body produces testosterone primarily in the testicles, with the remainder coming from your adrenal glands. Issues with these glands or related hormones can affect free testosterone levels.

Common culprits include:

• Obesity. Research links higher body fat percentages to lower levels of free testosterone. Enzymes inside fat cells convert testosterone into estrogen.

• Diabetes. Men with type 2 diabetes often have lower levels of free testosterone. 

• Hypothyroidism.  Low thyroid hormone levels may reduce testosterone and cause symptoms like fatigue, weight gain, and depression.

Lifestyle Habits

Your daily habits can also affect total and free testosterone levels. Here’s how:

• Diet. Eating too much processed food or unhealthy fats can lead to obesity, which lowers testosterone. A nutrient-rich diet with healthy fats and protein supports healthy testosterone production.

• Exercise. Moderate or intense physical activity may boost testosterone levels. But overtraining can have the opposite effect.

• Sleep. There’s a link between sleep deprivation and sleep disruptions and decreased levels of testosterone. 

• Stress. Chronic stress can raise levels of the stress hormone cortisol, which can suppress testosterone production.

Medications

The following drugs may lower testosterone:

• Anabolic steroids

• Opioids

• Corticosteroids

• Certain street drugs 

Steroid abuse, in particular, can shut down natural testosterone production. 

If your healthcare provider finds your testosterone levels are low, they may suggest a mix of lifestyle changes, treating underlying issues, and testosterone replacement therapy, (TRT). 

Here’s what treating low testosterone might involve:

• Lifestyle changes. Regular exercise, eating a nutrient-rich diet, prioritizing good sleep, and managing stress are all natural ways to boost your testosterone levels. 

• Treating underlying health conditions. Addressing issues like obesity, diabetes, or thyroid disorders can improve your testosterone levels and overall well-being. 

• Testosterone replacement therapy (TRT). TRT is the most common treatment for low testosterone levels. It involves hormone therapy through injections, gels, patches, or pellets. The goal is to restore testosterone to normal levels and reduce symptoms like fatigue, low libido, and muscle loss.

Learn more about the benefits and side effects of TRT. 

High testosterone levels can lead to issues like aggressive behavior. This might happen due to tumors in the pituitary gland or testicles. 

Very high testosterone levels can also happen if you take high doses of synthetic testosterone. 

Free testosterone is a form of testosterone that’s unbound from proteins and available for your body to use. It’s crucial for male health and wellness. 

Let’s recap what we know about free testosterone:

• You only have small amounts of free testosterone. Only about 2 percent of the total testosterone circulating in your body is in its free form.

• Your levels can be low for many reasons. Low testosterone is common in people with obesity and diabetes.

• You can address low levels. Treatment options include testosterone replacement therapy and lifestyle changes.

Think you might have low testosterone levels? Ask your healthcare provider about testing. They let you know if you might benefit from testosterone replacement therapy (TRT).