- Share of total
- A small percentage
- How it's obtained
- Measured directly or calculated from total testosterone and SHBG
- Test timing
- Early morning
Bound, loosely bound and free
Testosterone travels in the blood in three states. A large share is tightly bound to sex hormone–binding globulin (SHBG) and is largely unavailable. Another share is loosely attached to albumin and can be released when tissues need it. A small percentage circulates freely.
Labs describe this in a few ways. Total testosterone counts all of it. Free testosterone counts only the unbound part. Bioavailable testosterone counts free plus albumin-bound testosterone.
When free testosterone matters
Total testosterone is the standard first test and is enough for most people. Free testosterone becomes important when SHBG is unusually high or low, because then total testosterone can give the wrong impression. Common situations include:
- Higher SHBG with age, hyperthyroidism, liver disease or estrogen-containing medicines, which can make total testosterone look adequate while free testosterone is low.
- Lower SHBG with obesity, insulin resistance, type 2 diabetes or hypothyroidism, which can make total testosterone look low when free testosterone is closer to normal.
- A total testosterone result near the lower cutoff, where free testosterone helps decide whether the level is truly low.
Measured or calculated
Free testosterone can be measured directly with specialized lab methods, or calculated from total testosterone, SHBG and albumin using an established formula. Different methods can give noticeably different numbers, so results are read against the reference range of the lab that ran them. If you're comparing results over time, try to use the same lab and the same method.
Testing tips
Testosterone is highest in the morning, so blood is usually drawn early, roughly between 7 and 10 a.m. The Endocrine Society recommends confirming a low result with a repeat morning, fasting test, and the American Urological Association recommends two separate early-morning measurements before diagnosing deficiency. Acute illness, poor sleep, heavy alcohol use and some medicines, including opioids, can temporarily lower testosterone.
Free testosterone in women
Women make testosterone too, in much smaller amounts. Testing is mostly used to look for excess androgens, as in polycystic ovary syndrome, rather than to diagnose deficiency. Levels in women are much lower than in men, so results are interpreted with care and always alongside symptoms.
For men, a diagnosis of low testosterone rests on consistent lab results plus symptoms or signs. A free testosterone number on its own isn't a reason to start treatment.
Free testosterone: common questions
Is free testosterone more important than total testosterone?
Not usually. Total testosterone is the standard starting test; free testosterone adds information when SHBG is abnormal or total testosterone is borderline.
What is bioavailable testosterone?
It's free testosterone plus the testosterone loosely bound to albumin, which tissues can also use. It's ordered less often.
Why does SHBG matter?
SHBG binds testosterone tightly. When SHBG changes, the amount of free testosterone can change even if total testosterone stays the same.
What time should my test be?
Early morning, roughly 7 to 10 a.m., when levels are naturally highest. A low result should be repeated on another morning.
Can weight loss change free testosterone?
Weight loss can raise SHBG and total testosterone in men with obesity, so both numbers may shift. Your clinician will look at them together.
Do women ever need free testosterone tested?
Sometimes, mainly to evaluate signs of excess androgens such as unwanted hair growth, acne or irregular periods.
Sources
We cite government agencies, professional societies and peer-reviewed research. See our editorial policy.
- Testosterone Levels TestMedlinePlus (U.S. National Library of Medicine)
- SHBG Blood TestMedlinePlus (U.S. National Library of Medicine)
- Testosterone Therapy in Men with Hypogonadism: Clinical Practice GuidelineEndocrine Society
- Evaluation and Management of Testosterone Deficiency guidelineAmerican Urological Association