What does it measure?
Most testosterone travels bound to proteins, especially SHBG and albumin. Free testosterone may help clarify a result when total testosterone does not fit symptoms or SHBG is unusual. Laboratories may measure it directly with a suitable method or calculate it from other measurements. [1] [2]
Before the test
- Method
- Check whether the report gives a measured free value or an estimate calculated from other blood results.
- Timing
- Use the collection plan for the reason you are being tested; record hormone treatment and dose timing.
What can the result mean?
In men, a low result matters most when relevant symptoms and reliably low testosterone are both present. In women, a higher result may help investigate signs of androgen excess. The method, laboratory interval, and clinical question are essential; numbers from different assays may not be comparable. [1]
When is it useful, and how is it read?
In a symptomatic man with a borderline total result or a condition affecting SHBG, free testosterone may clarify whether the available hormone is low. The measurement method matters. A free result is interpreted with total testosterone and the clinical picture, not used alone as a treatment target. [3] [6]
What can affect it?
Age, illness, sleep, hormone medicines, and changes in binding proteins affect interpretation. For suspected testosterone deficiency in men, clinicians generally start with appropriately timed total testosterone measurements and use free testosterone selectively. Some direct free-testosterone assays are less reliable than validated measurement or calculation methods. [1] [2]
Binding proteins can change the picture
Most testosterone travels attached to proteins. A change in SHBG can make total testosterone and free testosterone tell different stories, which is why a reliable free result can help in selected situations. [3] [6]
Questions you might have
Why can total and free testosterone tell different stories?
Binding proteins affect how much testosterone is free. Free testosterone can add context when total testosterone does not fit the clinical picture. [3]
Are free-testosterone methods interchangeable?
No. Reliable measurement or a validated calculation matters. Compare the method and laboratory interval before comparing numbers. [1]
What comes next?
Ask which method your laboratory used and whether the result needs confirmation. Review total testosterone and SHBG together before considering treatment. Testosterone treatment is a separate clinical decision and can suppress sperm production, so fertility plans belong in that conversation. [1]
Was free testosterone measured or calculated, and is the method appropriate for my situation?
The reference interval and units on your report are part of understanding your result. Understand units, reference intervals, and treatment targets.
Sources & references
Medical references & original research
- Endocrine Society — Testosterone Therapy for Hypogonadism Guideline Resources
- Mayo Clinic Laboratories — Testosterone, Free (Dialysis) and Total, MS
- Mayo Clinic Laboratories — Testosterone, Total, Bioavailable, and Free, Serum
- Mayo Clinic Laboratories. Testosterone, Total, Bioavailable, and Free, Serum (TTFB).
- Bhasin S, et al. Testosterone Therapy in Men With Hypogonadism: An Endocrine Society Clinical Practice Guideline. Journal of Clinical Endocrinology & Metabolism. 2018;103:1715–1744.
Books we consulted
References that informed our explanations of the test and its context.
- Pagana KD, Pagana TJ, Pagana TN. Mosby’s Diagnostic and Laboratory Test Reference. 14th ed. Elsevier; 2019.
Practical-section sources accessed: .
For general health education. Interpreting your result and deciding on tests or treatment requires your health context and a qualified clinician.
