Free testosterone
The portion of testosterone in the blood that is not bound to proteins
Also known as
Definition
Most testosterone in the blood is bound to proteins, mainly SHBG (sex hormone-binding globulin) and albumin. Only the unbound, free portion is directly available to the body. Labs measure it directly, for example by equilibrium dialysis, or calculate it with a formula from total testosterone, SHBG and albumin. The value complements total testosterone, especially when SHBG is altered.
| Parameter | Value |
|---|---|
| Unit | pg/ml or pmol/l (depending on the lab) |
| Reference Range | Strongly dependent on the assay method, age and sex; each lab states its own range. The reference range on your own report is what counts. Discuss deviations with your doctor. |
Agree on your personal target range with your doctor.
Conversion via the molar mass of testosterone (288.4 g/mol): 1 pg/ml is about 3.47 pmol/l, 220 pmol/l is about 63 pg/ml. Some labs use ng/dl: 1 ng/dl = 10 pg/ml.
↓ What a low value means
In men, low free testosterone can be linked to reduced production in the testicles, disorders of the pituitary gland or hypothalamus, chronic conditions such as liver or kidney disease, or genetic causes such as Klinefelter syndrome. A high SHBG can lower the free portion even when total testosterone looks unremarkable. Testosterone also declines with age. In women, low levels are usual. A single value does not replace a medical assessment: discuss deviations with your doctor.
↑ What a high value means
In women, elevated testosterone can be linked to polycystic ovary syndrome (PCOS), ovarian disorders, or adrenal disorders such as congenital adrenal hyperplasia (CAH). In men, possible causes include the use of steroids or testosterone-raising products, adrenal disorders and, rarely, testicular tumors. A low SHBG can raise the free portion. Your doctor interprets the result together with symptoms and other values.
When to test
Blood is usually drawn in the morning between 7 and 10 a.m., when testosterone is highest, sometimes fasting. The Endocrine Society guideline foresees free testosterone in men mainly when total testosterone is near the lower limit or a condition alters SHBG. An abnormal result is usually confirmed with a second measurement. Never stop medication without talking to your doctor.
Frequently asked questions
What is the difference between free and total testosterone? +
Total testosterone includes bound and free hormone; free testosterone is only the unbound portion. There is also bioavailable testosterone: free testosterone plus the portion bound to proteins other than SHBG. Total testosterone is the most commonly measured.
Why is SHBG often measured as well? +
SHBG binds a large part of testosterone. If SHBG is high or low, total testosterone can look unremarkable while the free portion shifts. That is why free testosterone is often viewed together with SHBG and albumin or calculated from them.
How accurate is the measurement of free testosterone? +
It depends heavily on the method. MedlinePlus notes that free testosterone tests are often not very accurate. The Endocrine Society names equilibrium dialysis or calculation with an accurate formula as suitable approaches. Results over time are most comparable when they come from the same lab using the same method.
Which thresholds did the EMAS study use? +
In the European EMAS study of 3,369 men aged 40 to 79, late-onset hypogonadism was defined as at least three sexual symptoms together with total testosterone below 11 nmol/l and calculated free testosterone below 220 pmol/l (about 63 pg/ml). This is a study criterion, not a personal target. Your doctor interprets your own values.
Free testosterone
Sources
- MedlinePlus, National Library of Medicine: Testosterone Levels Test
- MedlinePlus, National Library of Medicine: Testosterone
- Bhasin S et al. (2018) Testosterone Therapy in Men With Hypogonadism: An Endocrine Society Clinical Practice Guideline. J Clin Endocrinol Metab.
- Wu FCW et al. (2010) Identification of late-onset hypogonadism in middle-aged and elderly men (EMAS). N Engl J Med.
Last updated: October 05, 2026
This information is for orientation only and does not replace medical advice. Reference ranges can vary by laboratory, method and country.