Free Testosterone Test

Most testosterone in your bloodstream is locked up, bound tightly to carrier proteins that prevent it from entering your cells. Only about 2 to 4 percent floats freely, and that sliver is what actually drives the effects you associate with testosterone: sex drive, muscle mass, bone density, energy, and mood. When your doctor orders a standard testosterone test, the result reflects the total amount in your blood, bound and unbound combined. That total number can look perfectly normal while the free fraction, the part doing the work, quietly falls short.

This gap between total and free testosterone is not rare. Conditions as common as weight gain, aging, and type 2 diabetes shift the balance of binding proteins in your blood, creating a false sense of security on a standard lab panel. Free testosterone testing closes that gap by measuring what your cells can actually access, giving you a much sharper picture of whether your body's androgen supply is truly meeting demand.

What Free Testosterone Actually Tells You

Free testosterone (the unbound fraction of total circulating testosterone) represents the biologically active hormone available to enter your cells, bind to androgen receptors, and switch on the genes that regulate muscle protein synthesis, bone mineral density, red blood cell production, and sexual function. In men, over 95% of testosterone is made by specialized cells in the testes called Leydig cells, under signals from the pituitary gland in the brain. Women produce smaller amounts from the ovaries and adrenal glands.

The rest of your testosterone, roughly 96 to 98 percent of it, rides through the bloodstream attached to two proteins. About 60 to 70 percent binds tightly to sex hormone binding globulin (SHBG), a carrier protein made by the liver. Another 25 to 35 percent binds loosely to albumin. These binding proteins act like a reservoir, holding testosterone in circulation but keeping it out of cells. Free testosterone is what's left over, and studies show it correlates more strongly with symptoms of testosterone deficiency (like low sex drive, fatigue, and reduced muscle mass) than total testosterone does.

When Total Testosterone Lies

The real value of free testosterone testing shows up when SHBG levels are not where they should be, because SHBG is the main variable that creates a wedge between your total number and your free number. Anything that raises SHBG (aging, liver disease, HIV infection, hyperthyroidism, or anticonvulsant medications like phenytoin) can push your total testosterone into the normal range while your free testosterone falls below it. You look fine on paper, but your cells are starving for androgen.

The reverse is equally misleading. Conditions that lower SHBG, especially obesity, type 2 diabetes, and hypothyroidism, can drag your total testosterone below the reference range while free testosterone stays normal. In the European Male Aging Study, men with low total testosterone but normal free testosterone (a pattern common in obese men with low SHBG) did not have the sexual or physical symptoms typically associated with testosterone deficiency. Men with low free testosterone, regardless of their total, did have those symptoms.

Mortality Risk

Large population studies have consistently linked lower free testosterone to higher risk of death from all causes. In a UK Biobank analysis of nearly 150,000 men followed for 11 years, those in the lowest fifth of calculated free testosterone had a 14% higher risk of dying from any cause compared to those in the highest fifth, after adjusting for age, waist circumference, and medical conditions. A separate UK Biobank analysis of about 155,000 men over roughly 9 years found an even more pronounced pattern: men in the highest fifth of free testosterone had a 20% lower risk of all-cause death compared to the lowest fifth.

There is a caveat. The Guangzhou Biobank Cohort Study of nearly 4,000 older Chinese men found the opposite for very high levels: men in the highest quartile of free testosterone had about 45% higher all-cause mortality and 88% higher cardiovascular mortality compared to those in the third quartile. This suggests the relationship may be U-shaped, with risk rising at both extremes. A 2025 dose-response meta-analysis of over 359,000 participants supports this interpretation for total testosterone.

Declining free testosterone over time may be an even stronger signal than a single reading. In the Concord Health and Ageing in Men Project, which tracked over 1,700 men aged 70 and older with repeat measurements over five years, progressive decline in calculated free testosterone remained associated with 13% higher all-cause mortality per unit of decline, even after adjusting for multiple health factors.

Heart Disease

The relationship between free testosterone and cardiovascular events is less straightforward than the mortality data might suggest. In the largest cardiovascular-specific analysis from UK Biobank (about 210,700 men followed for 9 years with nearly 8,800 cardiovascular events), men with lower calculated free testosterone actually had a slightly lower incidence of major cardiovascular events. Those in the lowest fifth had about a 10% lower risk compared to the highest fifth, even after full adjustment.

A meta-analysis of 37 observational studies covering over 43,000 men found that low endogenous testosterone predicted cardiovascular mortality (about 54% higher odds) and cardiovascular disease (about 17% higher odds). However, this meta-analysis primarily used total testosterone rather than free testosterone specifically. The divergence between total and free testosterone findings in cardiovascular outcomes remains an active area of research, and the direction of causality is debated.

Type 2 Diabetes

Low free testosterone predicts future diabetes risk. In the Massachusetts Male Aging Study, which followed over 1,100 men aged 40 to 70 for up to 10 years, each standard-deviation decrease in baseline free testosterone (about 4 ng/dL) was associated with a 58% higher odds of developing diabetes, after controlling for confounders. An individual-participant meta-analysis of over 20,600 men from nine cohorts found that diabetes was among the medical conditions most strongly associated with lower calculated free testosterone, with diabetic men averaging nearly 20 pmol/L lower free testosterone than non-diabetic men.

Cancer Associations

Higher free testosterone appears linked to modestly increased prostate cancer risk. In a UK Biobank analysis of nearly 200,000 men, each 50 pmol/L increase in free testosterone was associated with a 10% higher risk of prostate cancer. A separate analysis using a technique called Mendelian randomization (which uses genetic data to estimate causal effects) supported a causal connection specifically for aggressive prostate cancer, with each standard-deviation increase in genetically predicted free testosterone associated with a 23% higher odds.

Higher free testosterone was also associated with higher melanoma risk in UK Biobank (about 35% higher per 50 pmol/L increase) and with higher cancer-related mortality in a separate mortality analysis. On the other hand, low free testosterone was linked to higher all-cause mortality, which included cancer deaths. This suggests a nuanced relationship where the optimal zone for cancer risk may differ from the optimal zone for long-term survival.

Reference Ranges

Free testosterone reference ranges vary significantly depending on the laboratory, assay method, and how the lab defines its "normal" population. No harmonized reference range exists across labs, which makes comparing results between different labs unreliable. Always compare your results within the same lab and method over time.

The strongest population-based reference ranges for men come from a 2025 study using the gold-standard measurement method (equilibrium dialysis with mass spectrometry) in over 1,200 community-dwelling men. For healthy, non-obese men aged 18 to 39, the reference range was 184 to 749 pmol/L (2.5th to 97.5th percentile). Free testosterone declines by roughly 4.5 pmol/L per year of age. Obesity further reduces it: a BMI of 30 lowers free testosterone by about 14% compared to a BMI of 22, and a BMI of 35 lowers it by about 22%.

These ranges are drawn from published research. Your lab may use different assays and cutpoints. Compare your results within the same lab over time for the most meaningful trend.

Commercial lab lower limits for free testosterone typically range from 49 to 89 pg/mL (170 to 310 pmol/L) for men, but these thresholds vary considerably and are sometimes described even in the medical literature as somewhat arbitrary. Ethnicity also matters: South Asian men average roughly 30 pmol/L lower calculated free testosterone than White European or African Caribbean men, a difference partially explained by higher rates of visceral fat and insulin resistance. Black men in the U.S. tend to have about 15 pmol/L higher free testosterone than White men.

Why a Single Reading Can Fool You

Free testosterone has substantial day-to-day variability. The within-person biological coefficient of variation runs around 12 to 17%, meaning your level can naturally fluctuate by that amount from one draw to the next even if nothing about your health has changed. A reference change value of 52% is needed between two morning measurements to confirm a statistically real difference in testosterone. Put another way, your testosterone could drop by nearly half and still fall within the expected range of normal variation for a single individual.

About 30% of men whose first testosterone result falls in the low range will have a normal result on repeat testing. This is why the Endocrine Society recommends at least two separate fasting morning measurements before drawing any clinical conclusions. Do not make decisions based on a single number.

Several common situations can produce temporarily misleading results. Eating a meal before your blood draw can suppress testosterone by as much as 30% within one to two hours, with free testosterone dropping about 17%. Testosterone peaks in the early morning and falls roughly 15% by late afternoon (though this rhythm flattens after age 60). Acute illness is an especially powerful confounder: testosterone can plunge 43 to 58% within 24 hours of a heart attack, surgery, or traumatic brain injury, sometimes reaching levels typically seen after surgical castration. These drops are temporary and reflect the body's stress response, not true testosterone deficiency. Never test during or shortly after an acute illness.

Tracking Your Trend

A single free testosterone reading is a snapshot taken in a storm of biological noise. The real power of this test comes from serial tracking: measuring your level under consistent conditions (fasting, early morning, same lab) multiple times over months and years. This converts a noisy individual data point into a reliable trend line.

Over a 4.3-year follow-up in the UK Biobank, the concordance of calculated free testosterone was only 0.56 (where 1.0 would mean perfect agreement between readings), reflecting both real biological changes and measurement variability. That modest concordance means a single baseline reading tells you relatively little about where your free testosterone will sit a few years from now. Serial measurement does.

Get a baseline reading. If you are making lifestyle changes (losing weight, changing your exercise routine, adjusting medications), retest in 3 to 6 months to see whether those changes are moving the needle. After that, annual testing under the same conditions gives you the trajectory your body is actually on, rather than the one a single reading suggests. If your initial result is low, retest within 4 to 6 weeks before considering it confirmed.

Measurement Methods Matter

How your lab measures free testosterone significantly affects your result. The gold standard is a technique called equilibrium dialysis, which physically separates the free fraction from bound testosterone. It is accurate but technically demanding and not widely available. Most labs instead calculate free testosterone using a formula (the Vermeulen equation) that takes your total testosterone, SHBG, and albumin levels and estimates the free fraction mathematically. This calculated value is a reasonable approximation in most situations.

What you should avoid is a direct analog immunoassay for free testosterone. These are the cheapest and simplest tests to run, and some local labs offer them for convenience. They are seriously inaccurate and both the Endocrine Society and major endocrinology reviews recommend against their use. If you are ordering a free testosterone test, confirm with the lab that the result will be measured by equilibrium dialysis or calculated from total testosterone, SHBG, and albumin. A result from a direct analog assay is not clinically reliable.