Free testosterone is the small fraction of testosterone in your blood that isn’t attached to any proteins, making it the portion your body can actually use. Most testosterone travels through the bloodstream bound to carrier proteins, essentially locked up and unavailable. Only about 2% circulates freely, and that 2% is what enters your cells and drives the effects you associate with testosterone: muscle maintenance, sex drive, energy, and bone strength.
How Testosterone Travels in Your Blood
When a lab measures your “total testosterone,” it’s counting every molecule of testosterone in your blood, whether it’s available for use or not. The reality is that most of it is tied up. Roughly 44% is bound tightly to a protein called sex hormone-binding globulin (SHBG), which grips testosterone so firmly that it can’t detach and enter your cells. Another 50% is loosely bound to albumin, a common blood protein. About 4% attaches to a third carrier protein. That leaves approximately 2% completely unbound: your free testosterone.
The loosely bound portion (attached to albumin) can sometimes break free and become available to tissues. This is why you might also see the term “bioavailable testosterone” on a lab report, which combines the albumin-bound fraction with the free fraction. But free testosterone specifically refers to the unbound portion only.
Why the Free Fraction Matters More Than the Total
Testosterone is a fat-soluble hormone, which means it enters cells by slipping directly through cell membranes. It can only do this when it’s not attached to a carrier protein. This is called the free hormone hypothesis, and it applies to several hormones beyond testosterone, including thyroid hormones and cortisol. The principle is the same: only the unbound fraction is biologically active.
This distinction matters because your total testosterone can look perfectly normal on a lab report while your free testosterone is actually low. A large study published in the Journal of Clinical Endocrinology & Metabolism found that men with normal total testosterone but low free testosterone experienced the same problems as men whose total testosterone was also low. They reported more erectile dysfunction, fewer morning erections, reduced sexual thoughts, slower walking speed, and lower physical function scores. They also had lower hemoglobin levels, less lean muscle mass, and reduced bone density. In other words, their bodies were behaving as though they were testosterone-deficient, even though a standard total testosterone test wouldn’t have flagged anything.
Normal Free Testosterone Ranges
For adult men, the general reference range for free testosterone falls between 50 and 200 pg/mL according to Endocrine Society guidelines. A 2023 study using a standardized measurement method in healthy, non-obese men found a broader normative range of 66 to 309 pg/mL across all ages, with men between 19 and 39 falling between 120 and 368 pg/mL. The median value was 141 pg/mL.
These ranges vary depending on the lab and the method used to measure or calculate free testosterone, which is one reason your doctor interprets results in context rather than treating a single number as definitive. Age also plays a significant role. SHBG levels rise as you get older, binding up more testosterone and leaving less of it free. A 60-year-old man can have total testosterone in the normal range while his free testosterone has quietly dropped below the threshold where symptoms appear.
What Causes Free Testosterone to Drop
Anything that raises SHBG will lower your free testosterone, even if your body is still producing the same total amount. Aging is the most common cause, but several medical conditions push SHBG higher: overactive thyroid (hyperthyroidism), HIV infection, nephrotic syndrome, and conditions involving elevated growth hormone. Certain genetic variations in the SHBG gene can also lead to naturally higher levels of the binding protein.
Medications can have the same effect. Estrogen-containing products increase SHBG production in the liver. Some anti-seizure medications, including phenobarbital, phenytoin, and carbamazepine, stimulate liver enzymes that ramp up SHBG output. Tamoxifen, used in breast cancer treatment, has a similar estrogen-like effect on liver tissue. In all these situations, total testosterone may test within normal limits while free testosterone quietly falls.
On the flip side, obesity and insulin resistance tend to lower SHBG, which can raise free testosterone. In women, this shift is clinically significant because elevated free testosterone is one of the diagnostic markers for polycystic ovary syndrome (PCOS). International guidelines specify that PCOS can be identified through elevated total or free testosterone, and calculated free testosterone or the free androgen index are accepted tools for confirming biochemical hyperandrogenism.
How Free Testosterone Is Measured
Free testosterone is tricky to measure directly. The gold standard is a lab technique called equilibrium dialysis, but it’s expensive and not widely available. Most labs instead calculate free testosterone using a formula. The most common approach, developed by Vermeulen, uses your total testosterone level and your SHBG level to estimate the free fraction through a mathematical model based on how proteins bind to hormones. Albumin is also part of the equation, though labs often use a standard average albumin value rather than measuring it separately, since albumin levels need to change significantly before they meaningfully affect the result.
You may also see something called the free androgen index (FAI) on lab reports. This is a simpler ratio: total testosterone divided by SHBG, multiplied by 100. It’s a rougher estimate but is commonly used, particularly in evaluating women for PCOS.
When Free Testosterone Testing Is Most Useful
A total testosterone test is usually the first step. Free testosterone becomes important when your total level is borderline or normal but you’re still experiencing symptoms like low energy, reduced sex drive, erectile problems, loss of muscle mass, or difficulty with physical activities you used to handle easily. It’s also valuable when there’s reason to suspect your SHBG levels are unusually high or low, since that directly skews how much of your total testosterone is actually available.
For men over 50, testing free testosterone alongside total testosterone gives a much clearer picture of hormonal status. For women being evaluated for PCOS or other conditions involving excess androgens, free testosterone or calculated indices help confirm whether elevated androgens are truly driving symptoms like acne, hair thinning, or irregular periods. In both cases, the free fraction tells you what your body is actually working with, not just what’s circulating in name only.