For healthy adult men of all ages, free testosterone typically falls between 66 and 309 pg/mL when measured using the gold-standard equilibrium dialysis method. Younger men (ages 19 to 39) run higher, with a normal range of 120 to 368 pg/mL. These numbers come from a large reference study of healthy, nonobese men and represent the 2.5th to 97.5th percentiles, meaning 95% of healthy men fall within them.
But interpreting your results isn’t as simple as checking whether you’re inside the range. The testing method your lab used, your age, your weight, and how tightly a specific protein binds your testosterone all shape what your number actually means.
What “Free” Testosterone Actually Is
Only about 2% of the testosterone in your blood is truly free, meaning it floats unattached and can enter cells directly. The rest is bound to carrier proteins: roughly 44% to a protein called SHBG (sex hormone-binding globulin) and about 50% to albumin. Testosterone bound to SHBG is locked up tightly and essentially inactive. Testosterone bound to albumin is held loosely and can detach more easily, which is why some doctors refer to free testosterone plus albumin-bound testosterone together as “bioavailable” testosterone.
Free testosterone matters because it’s the fraction that can actually interact with tissues throughout your body, influencing muscle mass, bone density, energy, mood, and sexual function. Two men can have identical total testosterone levels but very different free testosterone levels depending on how much SHBG each one produces.
Normal Ranges by Age
Free testosterone declines with age, and the drop is steeper than the decline in total testosterone because SHBG levels tend to rise as men get older. More SHBG means more testosterone gets bound up, leaving less of it free.
Using equilibrium dialysis, the median free testosterone for men ages 19 to 39 is about 190 pg/mL, with the normal range spanning 120 to 368 pg/mL. Across all adult ages, the median drops to 141 pg/mL, and the full range widens to 66 to 309 pg/mL to account for the natural decline in older men.
Commercial labs often use their own calculated reference intervals. Quest Diagnostics, for example, lists 46 to 224 pg/mL for men ages 18 to 69, and 6 to 73 pg/mL for men 70 to 89. These numbers are lower than the dialysis-based reference study because they’re generated using a different calculation method. That difference in methodology is one of the biggest sources of confusion when people try to compare their results to numbers they find online.
Why Your Lab Method Changes Everything
There are three common ways to measure or estimate free testosterone, and they do not produce the same numbers.
- Equilibrium dialysis is considered the gold standard. It physically separates the free fraction from bound testosterone in a lab. The reference ranges cited above (66 to 309 pg/mL for all ages) come from this method.
- Calculated free testosterone uses a formula that takes your total testosterone, SHBG, and albumin levels and estimates the free fraction mathematically. It correlates well with dialysis but tends to slightly overestimate the result. Many labs use this approach because it’s cheaper and faster.
- Direct immunoassay (analog method) measures free testosterone directly but produces values roughly one-seventh of what equilibrium dialysis shows. If your lab used this method, the reference range printed on your report will look dramatically lower, and comparing those numbers to dialysis-based ranges would be meaningless.
Each method requires its own set of reference values. The single most important thing you can do when reading your results is check which method your lab used and compare your number only to the reference range printed on that specific report.
When Free Testosterone Testing Helps
Most clinical guidelines start with total testosterone to evaluate whether levels are low. The American Urological Association recommends a cutoff of 300 ng/dL for total testosterone as the threshold supporting a diagnosis of testosterone deficiency. Several other major medical societies use cutoffs ranging from 230 to 350 ng/dL.
Free testosterone becomes most useful in a gray zone. In an analysis of over 3,200 men, a free testosterone measurement added no diagnostic value when total testosterone was already clearly low (below 231 ng/dL). But when total testosterone fell between 230 and 317 ng/dL, checking free testosterone helped clarify whether the man was truly deficient or not. This is because conditions that raise SHBG, such as aging, liver disease, hyperthyroidism, or certain medications, can push total testosterone into a borderline range while free testosterone drops low enough to cause symptoms.
Obesity works in the opposite direction. Excess body fat tends to lower SHBG, which can make total testosterone appear low even when free testosterone is adequate. In both scenarios, measuring free testosterone gives a clearer picture of what’s biologically available.
What Affects Your SHBG (and Therefore Your Free T)
Because SHBG is the main gatekeeper determining how much testosterone stays free, anything that shifts SHBG will shift your free testosterone in the opposite direction. SHBG tends to increase with age, low calorie intake, hyperthyroidism, liver disease, and certain anticonvulsant medications. It tends to decrease with obesity, insulin resistance, type 2 diabetes, and hypothyroidism. Even genetics play a role: certain SHBG gene variants can raise free testosterone by over 20% compared to other variants, independent of total testosterone levels.
Getting an Accurate Test
Testosterone levels follow a daily rhythm, peaking between about 5:30 and 8:00 a.m. and hitting their lowest point roughly 12 hours later. This pattern holds for free and bioavailable testosterone just as it does for total testosterone. Getting your blood drawn in the morning gives you a reading at or near your peak and is the standard recommendation for both young and older men.
A single low reading isn’t enough for a diagnosis. Levels fluctuate day to day, so guidelines generally call for at least two morning samples on separate days before drawing conclusions. Fasting isn’t strictly required, but eating a large meal before the draw can temporarily suppress testosterone, so many clinicians suggest fasting or eating lightly.
Symptoms of Low Free Testosterone
The symptoms that prompt most men to get tested include reduced sex drive, erectile difficulty, persistent fatigue, loss of muscle mass or strength, increased body fat (particularly around the midsection), depressed mood, and difficulty concentrating. None of these symptoms are specific to low testosterone. They overlap with dozens of other conditions, which is exactly why blood testing matters rather than relying on symptoms alone.
Some men with borderline total testosterone and clearly low free testosterone experience these symptoms and see improvement with treatment, while men with the same total testosterone but normal free testosterone may not. This is the core reason free testosterone testing exists: it can explain why some men feel symptomatic even when their total testosterone looks acceptable on paper.