What It Is & What It Does for You

Welcome to Testosterone HQ—Men’s Health‘s guide to the exciting, complicated, and revolutionary world of testosterone. For everything you need to know about T, click here.

HORMONE EXPERTS AGREE on a few things: Low T can cause symptoms including ED, low libido, and exhaustion. (Low T is often defined as anything under 300 nanongrams per deciliter (ng/dL).

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They say that your total testosterone levels will shed the most light on whether you’re dealing with hypogonadism (the clinical term for low T). That’s the amount of bound testosterone and free testosterone that you have. What we know less about: Just how important those free testosterone numbers are.

“Total T levels do seem to correlate with a variety of important health-related outcomes, but little is actually known about free T—the research on it isn’t very strong,” explains andrology expert Charles Welliver, MD, associate professor of urology at Albany Medical College.

What Does Free T Tell You?

You have two main types of testosterone in your body: unbound (free T) and bound. Unbound T freely circulates, and because it’s untethered, it’s the only form of testosterone that can actually have an end-effect on your organs and muscles.

“Free T is what allows you to put on muscle, to keep your libido up, to help your erections, potentially even helping to influence your mood,” says Rajiv Jayadevan, MD, assistant clinical professor of urology and male fertility expert at The Men’s Clinic at UCLA Health.

Yet free T actually only comprises about 2 percent of your total testosterone, explains Dr. Welliver. The majority of your testosterone is bound tightly to proteins called sex hormone-binding globulins (SHBG), which sequester the free T and take it out of action so it can’t have any end effect. A smaller amount of the hormone is also lightly bound to a protein called albumin.

So if you just focus on your free testosterone numbers, you’re discounting the vast majority of the testosterone in your body.

What Affects Your Free T?

Your free T levels are tanked mostly by obesity and high amounts of SHBG, Dr. Welliver explains. Those SHBG levels are affected by everything from weight loss and weight gain to thyroid issues, infections, and simple aging.

That means trying to raise your testosterone is a layered game of increasing your free T and lowering your SHBG. A healthy diet can help, so can regular exercise. Of course, testosterone replacement therapy (TRT) also raises your total T levels, and helps reverse the effects of hypogonadism.

Should You Get Your Free T tested?

The number one thing you want to get tested if you’re symptomatic is your total testosterone levels, which includes all forms of testosterone in your body: unbound free T, SHBG-bound T, and albumin-bound T. This collective number is what paints the full picture of how your hormone levels are interacting with your body.

That being said, it can be worthwhile to ask your doc to also test your free T, bioavailable T, and SHBG levels.

These sub-numbers can help shed light on why you may be experiencing symptoms when everything looks normal with the total T. “We do sometimes initiate treatment on men with a low calculated free T if the total T is borderline,” Dr. Welliver says.

For instance, a 2021 study in Infection found that among HIV-positive men (for whom low T is very common), only 10.6 percent met diagnosis for hypogonadism based solely on total testosterone levels. But when considering calculated free testosterone, luteinizing hormones, and sex hormone-binding globulin levels, up to 20.2 percent of men would qualify as having subsets of the condition, like overt or compensated hypogonadism.

That’s also why it’s best to get your T levels tested through your doctor instead of just going out and getting one on your own. A physician can help you dive into your individual circumstances and help you get the right treatment for your body.