Testosterone lab tests: total vs free vs bioavailable
Three numbers, one hormone, and why physicians often look at more than one of them.
TL;DR
- Total testosterone counts every molecule in circulation. It is the usual first test.
- Free testosterone (about 2 to 4 percent of total) is the part the body can use most readily.
- Bioavailable testosterone is free plus loosely bound. Free or bioavailable matters most when SHBG is unusual.
- Test early in the morning, and confirm a low result on a second morning.
What it is
Testosterone in the bloodstream travels three ways. Most of it is either tightly bound to a carrier protein called SHBG (in plain English: sex hormone binding globulin, a protein the liver makes that grips testosterone like a clenched fist) or loosely bound to albumin. Only 2 to 4 percent floats free (Endocrine Society guideline, 2018). Think of SHBG-bound testosterone as money in a CD: present but not accessible. Albumin-bound is money in checking. Free testosterone is cash in your wallet.
The three tests, in plain words
- Total testosterone measures bound and free testosterone together. It is the most common type of testosterone blood test (MedlinePlus, Testosterone levels test).
- Free testosterone measures only the unattached form.
- Bioavailable testosterone measures free testosterone plus testosterone attached to proteins other than SHBG, mainly albumin (MedlinePlus, same page).
- SHBG is often ordered alongside total testosterone. A total test can't separate free from bound testosterone, so the SHBG result helps estimate how much is free (MedlinePlus, SHBG test).
How it works
Only free and loosely bound testosterone can leave the bloodstream and act on cells. The total number tells you how much testosterone exists. The free or bioavailable number tells you how much is available to do work. When SHBG is high, total testosterone can look normal while free testosterone is functionally low. The opposite happens with low SHBG: total can look low while free is fine.
The Endocrine Society lists common reasons SHBG shifts. Obesity, diabetes, an underactive thyroid, and some steroid medicines lower it. Aging, liver disease, an overactive thyroid, some seizure medicines, and estrogens raise it. In those situations, and when total testosterone sits in a borderline zone of roughly 200 to 400 ng/dL, the guideline recommends checking free testosterone (Endocrine Society guideline, 2018). The 2018 Endocrine Society guideline summary is a good starting point.
Which test should you ask for?
For most men, total testosterone on an early-morning draw comes first. Add SHBG and free testosterone when total is borderline, when one of the conditions above applies, or when symptoms and the total don't match. How free testosterone is measured matters. The Endocrine Society recommends equilibrium dialysis (the reference method) or an accurate calculation, and says direct analog free testosterone immunoassays are inaccurate and should not be used. Ask which method your lab uses.
What a testosterone blood test involves
- Time of day. The sample is usually taken between 7 and 10 a.m., when levels are typically highest (MedlinePlus).
- Fasting. The Endocrine Society recommends a fasting morning sample. MedlinePlus notes you may need to fast for several hours.
- Two mornings, not one. The American Urological Association strongly recommends two total testosterone measurements on separate occasions, both early in the morning (AUA guideline, 2018, reviewed 2024).
- Medicines. Tell your provider about everything you take, including supplements. Don't stop any medicine unless your provider tells you to.
- At home or at a lab. Morning timing is the main thing to get right with a home kit. Our guide to at-home blood test accuracy covers the rest. Use the same lab for the repeat test.
What usually comes next after a low result
A confirmed low number is the start of a workup, not the end. The AUA recommends measuring luteinizing hormone (LH) and prolactin in men with low testosterone, which helps show where the problem starts. Before any testosterone therapy, it recommends checking hemoglobin and hematocrit, and PSA in men over 40 (AUA guideline). Other causes of tiredness and low drive deserve a look too. Here is a map of fatigue in men over 40.
What results can and cannot tell you
A reference range describes a population, not your personal target. Labs also differ. The harmonized 264 to 916 ng/dL range applies to CDC-certified assays, and labs that are not certified may use quite different ranges (Endocrine Society guideline, 2018). A number alone is not a diagnosis. The Endocrine Society recommends against routine testosterone screening of men in the general population. Testing is meant for men with symptoms or signs, such as low sex drive, fewer spontaneous erections, hot flushes, or a fracture from a minor fall. If your total looks fine but your free number is low, read what low free testosterone means.
Questions to ask any provider before you pay
- Will you test total testosterone on an early-morning draw?
- Will you add SHBG and free testosterone, and which free testosterone method does the lab use?
- Is the lab's total testosterone assay CDC-certified?
- Will a low result be repeated on a second morning?
- Who reviews the results, and will they check LH and prolactin if I'm low?
- Will anything be prescribed only if my labs and symptoms support it?
Who asks about it
People ask about the different testosterone tests after they get one number from a primary-care lab draw, the result lands in the gray zone, and a physician suggests follow-up tests. Some ask after a friend mentions "my free T was low even though my total was normal." Both situations are real and common.
What the research says
The Endocrine Society's harmonized reference range for total testosterone in adult men is 264 to 916 ng/dL, standardized to the CDC and based on healthy, nonobese young men (Endocrine Society guideline, 2018). The AUA uses a total testosterone below 300 ng/dL as a reasonable cut-off (AUA guideline). Free testosterone results vary by lab and assay method. Direct immunoassay can give different results than equilibrium dialysis, which is the reference method.
What to know before considering it
Testosterone testing should be done early in the morning and ideally on two separate days, because levels move through the day and from day to day. A single low result is not a diagnosis. Symptoms should be present alongside confirmed low readings before testosterone therapy is considered, per Endocrine Society guidance.
The Halftime POV
One number rarely tells the full story. Two numbers usually do. The right read puts the result next to the symptoms, not the symptoms next to the number, because population reference ranges are not individualized targets.
How Halftime does this
Halftime Health membership includes an onboarding blood panel you collect at home. A licensed physician reviews your labs and decides whether a protocol is appropriate for you. If a protocol is prescribed, a licensed 503A compounding pharmacy fills it. You retest later to see what changed. Read more about how Halftime blood testing works.
Related reading:
- Testosterone panels: total, free, and SHBG
- Low testosterone diagnosis: the workup
- Free testosterone levels: what low free T means
- Fatigue in men over 40: a hormonal and metabolic map
- Men's midlife panel 2026
FAQ
Q: What is the difference between total and free testosterone? A: Total testosterone is every testosterone molecule in your blood. Free testosterone is the small fraction (about 2 to 4 percent) not bound to a carrier protein, the portion the body can use most readily. Bioavailable testosterone is free testosterone plus the loosely bound fraction, which can detach when needed.
Q: Which testosterone number matters most? A: Physicians usually start with total testosterone. Free or bioavailable testosterone is added when total is borderline or when SHBG (sex hormone binding globulin) is likely high or low, because an unusual SHBG can hide a meaningful problem in the total number alone.
Q: What is a normal testosterone level? A: For total testosterone, the Endocrine Society cites a harmonized range of 264 to 916 ng/dL for CDC-certified assays, based on healthy, nonobese young men. Free testosterone ranges vary by lab and method. Reference ranges describe populations, not personal targets, and symptoms matter alongside the number.
Q: When should I get a testosterone blood test? A: Early in the morning, usually between 7 and 10 a.m., when levels are typically highest. A low result should be confirmed with a second early-morning test on a different day before anyone acts on it.
Q: Do I need to fast for a testosterone blood test? A: Often, yes. The Endocrine Society recommends a fasting morning sample, and MedlinePlus notes you may need to fast for several hours. Follow your lab's instructions, and don't stop any medicine unless your provider tells you to.
Disclaimer
This article is educational and is not medical advice. Compounded medications are not FDA-approved. Clinical outcomes depend on individual factors and require physician evaluation. Results vary.
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We share what we learn along the way: the research, the regulations, the real-world trade-offs. Get started with Halftime Health.
Sources
- Bhasin S et al. Testosterone Therapy in Men With Hypogonadism: An Endocrine Society Clinical Practice Guideline. J Clin Endocrinol Metab, 2018
- Endocrine Society Clinical Practice Guideline
- American Urological Association: Testosterone Deficiency Guideline (2018, reviewed 2024)
- MedlinePlus: Testosterone levels test
- MedlinePlus: SHBG blood test
Frequently asked questions
What is the difference between total and free testosterone?
Total testosterone is every testosterone molecule in your blood. Free testosterone is the small fraction (about 2 to 4 percent) not bound to a carrier protein, the portion the body can use most readily. Bioavailable testosterone is free testosterone plus the loosely bound fraction, which can detach when needed.
Which testosterone number matters most?
Physicians usually start with total testosterone. Free or bioavailable testosterone is added when total is borderline or when SHBG (sex hormone binding globulin) is likely high or low, because an unusual SHBG can hide a meaningful problem in the total number alone.
What is a normal testosterone level?
For total testosterone, the Endocrine Society cites a harmonized range of 264 to 916 ng/dL for CDC-certified assays, based on healthy, nonobese young men. Free testosterone ranges vary by lab and method. Reference ranges describe populations, not personal targets, and symptoms matter alongside the number.
When should I get a testosterone blood test?
Early in the morning, usually between 7 and 10 a.m., when levels are typically highest. A low result should be confirmed with a second early-morning test on a different day before anyone acts on it.
Do I need to fast for a testosterone blood test?
Often, yes. The Endocrine Society recommends a fasting morning sample, and MedlinePlus notes you may need to fast for several hours. Follow your lab's instructions, and don't stop any medicine unless your provider tells you to.
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