What Is Free Testosterone? Total Testosterone vs Free Testosterone Explained

If you have had a testosterone blood test, you may have seen results for total testosterone, free testosterone or SHBG. They are related, but they do not mean the same thing. Total testosterone measures testosterone in the blood overall, while free testosterone estimates the small proportion that is not bound to carrier proteins (like SHBG and Albumin). Looking at the results in context can help a clinician interpret them, particularly when SHBG is outside the expected range.

What is testosterone?

Testosterone is a hormone involved in several functions, including sexual development and function, sperm production, muscle and bone maintenance, and the production of red blood cells. Testosterone levels vary between individuals and can also vary within the same person over time.

A blood result is only one part of an assessment. Symptoms and signs, medical history, medicines, the timing of the sample and the laboratory method may all affect how a result is interpreted. Symptoms alone do not establish testosterone deficiency, and a single result should not be treated as a diagnosis.

What is total testosterone?

Total testosterone is the overall amount of testosterone measured in a blood sample. It includes testosterone that is bound to proteins as well as the small amount circulating freely.

Most circulating testosterone is bound to either sex hormone-binding globulin (SHBG) or albumin. SHBG binds testosterone relatively tightly; albumin binds it more weakly. The total testosterone result therefore reflects more than the unbound hormone alone.

Total testosterone is commonly used in the initial biochemical assessment. If the result is low or borderline, it needs to be interpreted alongside the clinical picture and relevant guidance rather than in isolation.

What is free testosterone?

Free testosterone is the small proportion of testosterone circulating unbound to SHBG or albumin. It accounts for roughly 2–3% of circulating testosterone, although the proportion varies.

Because free testosterone is not bound to these carrier proteins, it can provide additional information in selected situations. It is not automatically a better or more important result for every person. Total and free testosterone are related but are not interchangeable.

Total vs free testosterone: what is the difference?

Why does SHBG matter?

SHBG is a protein that binds testosterone in the bloodstream. Differences in SHBG can affect the relationship between total and free testosterone.

SHBG levels can be influenced by a range of factors. For example, levels may be higher with increasing age or an overactive thyroid, and lower in some people with obesity or an underactive thyroid. Other health conditions and medicines can also influence results.

This matters because two people with similar total testosterone results may have different free testosterone estimates if their SHBG levels differ. In some circumstances, total testosterone may appear within the laboratory reference range while free testosterone is lower than expected; in other circumstances, the pattern may be different. Neither pattern should be interpreted without appropriate clinical context.

A result described as “high SHBG” or “low free testosterone” is not, by itself, a diagnosis or a reason to assume a particular treatment is needed. It is a reason to interpret the findings carefully and consider whether further assessment is appropriate.

When might a free testosterone result be useful?

Free testosterone may be particularly useful when total testosterone is close to the lower end of the reference range or when a condition or factor affecting SHBG could make total testosterone harder to interpret.

Not everyone needs a free testosterone measurement. The decision depends on the initial result, symptoms and signs, SHBG and other relevant clinical factors. A clinician can explain whether additional testing would help answer the question being investigated.

Can you diagnose testosterone deficiency from free testosterone alone?

No single testosterone measurement should be used in isolation to diagnose testosterone deficiency. Assessment generally considers compatible symptoms or signs alongside consistently low biochemical results, with repeat testing and appropriate sampling conditions where indicated.

Testosterone varies during the day and can be affected by acute illness and other factors. Clinicians should interpret the result using the relevant laboratory reference range and applicable UK guidance. A result outside a reference range does not automatically explain a person's symptoms, and a result within range does not necessarily settle every clinical question.

If you are concerned about symptoms or a blood-test result, discuss it with a doctor rather than trying to diagnose yourself from one number or an online range.

What should you do if your total and free testosterone results differ?

If your results seem inconsistent, useful questions to discuss with your doctor include:

• Was the sample taken at an appropriate time and under suitable conditions?
• Does the result need to be repeated?
• Was SHBG measured, and could it help explain the pattern?
• How was free testosterone measured or calculated?
• Are there health conditions, medicines or other factors that may affect the results?
• Do the results fit with the symptoms, signs and wider medical history?

Do not compare numbers without checking units, reference ranges and laboratory methods. If repeat testing or additional investigations are appropriate, your doctor can explain why and how they will inform the assessment.

Frequently asked questions

Is free testosterone the same as bioavailable testosterone?

No. Free testosterone is the unbound fraction. Bioavailable testosterone usually refers to free testosterone plus testosterone weakly bound to albumin.

Is free testosterone more important than total testosterone?

Not universally. They provide different information. Free testosterone can add context in selected cases, such as when total testosterone is borderline or SHBG is altered, but interpretation depends on the clinical situation and testing method.

Can SHBG make total testosterone misleading?

SHBG can affect the relationship between total and free testosterone. This is one reason clinicians may consider SHBG and, where appropriate, free testosterone when interpreting a result.

Does a low free testosterone result mean I have testosterone deficiency?

Not by itself. A clinician needs to interpret it alongside symptoms or signs, other results, the testing method and relevant guidance. Repeat testing may be needed.

Should every man have free testosterone tested?

Not necessarily. The appropriate tests depend on the reason for assessment and the initial findings. A doctor can advise whether free testosterone would add useful information.

The key point

Total testosterone, free testosterone and SHBG are related but distinct measures. Free testosterone can help clarify selected results, especially where total testosterone is borderline or SHBG may be affecting interpretation. Neither a symptom list nor a single laboratory result can establish testosterone deficiency on its own. The meaningful next step is an assessment that considers the result alongside the person, not the number alone.

Concerned about symptoms? Speak to a doctor – your GP or us.

General information, not individual medical advice. Treatment is prescribed only if clinically appropriate after assessment.

Sources and references

1. Bhasin S, et al. Testosterone Therapy in Men With Hypogonadism: An Endocrine Society Clinical Practice Guideline. Journal of Clinical Endocrinology & Metabolism (2018). https://academic.oup.com/jcem/article/103/5/1715/4939465
2. Testosterone Deficiency — Establishing a Biochemical Diagnosis (review). https://pmc.ncbi.nlm.nih.gov/articles/PMC4975356/
3. Mulhall JP, et al. Evaluation and Management of Testosterone Deficiency: AUA Guideline. Journal of Urology (2018). https://www.auajournals.org/doi/10.1016/j.juro.2018.03.115