What Does This Test Measure?
Most testosterone circulating in the blood is not available to tissues. Roughly 40 to 60 percent is bound tightly to sex hormone binding globulin, and most of the remainder is bound loosely to albumin. Only about 1 to 3 percent circulates completely unbound. That unbound fraction is free testosterone, and together with the loosely albumin-bound portion it makes up bioavailable testosterone; the part that can actually enter cells and act.
This distinction is the entire point of the test. Total testosterone measures everything, bound and unbound. When SHBG is unusually high or low, total testosterone stops reflecting what tissues experience, and free testosterone becomes the more meaningful number.
Free testosterone is usually calculated from total testosterone, SHBG and albumin rather than measured directly. Direct analogue immunoassays are widely available but unreliable, and calculated values or equilibrium dialysis are preferred.
In adult men, roughly 50–210 pg/mL, or about 1.5–3 percent of total testosterone, with a clear decline across adult life. In women, approximately 1–8.5 pg/mL. Ranges vary widely between calculation formulas and assays, so serial results should come from the same laboratory using the same method. Because levels peak in the early morning, the reference intervals assume a morning sample.
Why Your Doctor Ordered This Test
The main reason is a total testosterone that does not fit the clinical picture, typically a man with clear symptoms of low testosterone whose total result sits in the low-normal range, or an apparently low total in someone with no symptoms at all.
It is particularly useful whenever SHBG is likely to be abnormal: obesity, type 2 diabetes and nephrotic syndrome lower SHBG, while ageing, hyperthyroidism, liver disease, anticonvulsants and oral estrogen raise it. In women it is measured when investigating androgen excess such as hirsutism, acne and irregular periods, where free testosterone is more sensitive than total.
What High Free Testosterone Means
In men, a high free testosterone most often reflects exogenous testosterone or anabolic steroid use, and is usually accompanied by suppressed LH and FSH. It can also arise from a low SHBG, in which case total testosterone may be unremarkable while the free fraction is disproportionately high.
In women, raised free testosterone is the key biochemical marker of androgen excess. Polycystic ovary syndrome is by far the commonest cause, and free testosterone is frequently raised there even when total testosterone is normal, because the associated insulin resistance suppresses SHBG. Other causes include congenital adrenal hyperplasia, and, when levels are markedly raised with rapid onset of virilisation, an androgen-secreting ovarian or adrenal tumour, which requires prompt investigation.
What Low Free Testosterone Means
Low free testosterone with symptoms such as reduced libido, erectile difficulty, fatigue, loss of muscle mass, low mood and reduced morning erections supports a diagnosis of hypogonadism. The next step is always LH and FSH, which separate testicular failure from a pituitary cause.
A common and easily missed pattern is normal total testosterone with low free testosterone caused by a high SHBG. This occurs with ageing, hyperthyroidism, liver disease, anticonvulsant medication and oral estrogen. Relying on total testosterone alone in these situations produces false reassurance in a genuinely hypogonadal patient.
Obesity and type 2 diabetes usually produce the opposite pattern, low SHBG with a low total testosterone but a better-preserved free level, and weight loss frequently improves both.
How to Prepare and What Affects the Result
Sample between 7am and 10am, ideally fasting, because testosterone follows a strong daily rhythm and an afternoon sample can read 20 to 25 percent lower. Because a single low value is common in healthy men, diagnosis requires at least two low morning samples on separate days.
SHBG and albumin must be measured on the same sample if free testosterone is to be calculated. Acute illness, poor sleep, recent heavy exercise and significant stress all lower results and are worth avoiding beforehand. Testosterone therapy, anabolic steroids, opioids, corticosteroids and biotin supplements all affect results and should be declared.
Common Misinterpretations
The most consequential error is accepting a normal total testosterone as proof that testosterone status is fine. With a high SHBG the free fraction can be clearly low while the total looks reassuring, and the patient is told nothing is wrong despite genuine deficiency.
The second is using a direct free testosterone immunoassay. These analogue methods are widely offered but poorly correlated with equilibrium dialysis, particularly at low concentrations, and calculated free testosterone from total, SHBG and albumin is considerably more reliable.
The third is diagnosing hypogonadism from a single sample, which ignores both the daily rhythm and day-to-day variability. Two morning measurements are the accepted minimum.
Frequently Asked Questions
Why is my free testosterone low when my total is normal?
Almost always because SHBG is high. SHBG binds testosterone tightly and keeps it away from tissues, so a raised SHBG means more of your total is locked up and less is available. Ageing, hyperthyroidism, liver disease, anticonvulsants and oral estrogen all raise SHBG. In this situation the free value reflects how you feel far better than the total does.
Should free testosterone be measured or calculated?
Calculated is generally better in routine practice. Direct analogue immunoassays are widely available but correlate poorly with the reference method, especially at low levels. Calculating free testosterone from total testosterone, SHBG and albumin is more reliable, while equilibrium dialysis remains the gold standard where precision is critical.
Does one low result mean I have low testosterone?
No. Testosterone varies with time of day, sleep, illness, stress and recent exercise, and healthy men occasionally record a low morning value. Diagnosis requires at least two low morning samples taken on separate days, interpreted alongside symptoms and with LH and FSH to establish the cause.
Why does free testosterone matter in women?
Because it is more sensitive than total testosterone for detecting androgen excess. In polycystic ovary syndrome the associated insulin resistance lowers SHBG, which raises the free fraction while total testosterone may remain within range. A woman with hirsutism, acne and irregular cycles can therefore have a normal total and a clearly raised free testosterone.
Can losing weight change my result?
Frequently, yes. Excess body fat increases conversion of testosterone to estradiol and lowers SHBG, and the associated insulin resistance suppresses the whole axis. Meaningful weight loss commonly raises both total and free testosterone without any hormone treatment, which is why it is usually addressed before considering therapy.
Related Markers: Read Together for Full Context
BloodWorker reads lab values together, not in isolation. These related markers provide essential context for interpreting your Free Testosterone result:
Upload Your Bloodwork for Cross-Referenced Analysis
BloodWorker reads all of your markers together , not one at a time , to surface the patterns a thoughtful clinician would notice. Upload your labs and see what the full picture reveals.
Get Started