Hormone Health
Free vs Total Testosterone: What the Blood Test Numbers Mean
Total testosterone is the screening test, but free testosterone tells a different part of the story when the total is borderline or binding proteins are off. Here is what each number means and when it matters.
Reviewed by the clinical team at Zest Regen Health · October 28, 2026 · 8 min read
Content managed by Phil Kim, Lucky Consulting Group
Total testosterone is the first test used to evaluate low testosterone symptoms, but it does not tell the whole story. Free testosterone is the small fraction of testosterone not bound to proteins, and the Endocrine Society's 2018 clinical practice guideline recommends measuring it when total testosterone is borderline or when a condition that changes sex hormone-binding globulin (SHBG) is present. At Zest Regen Health in Columbia, Maryland, hormone testing is ordered and interpreted by the clinical team, so the right tests are chosen for the right reason.
What is the difference between total and free testosterone?
Total testosterone measures everything in the blood: the fraction bound to SHBG, the fraction loosely bound to albumin, and the small free fraction. Free testosterone is exactly what the name says, the tiny portion not bound to any protein, roughly 1 to 3 percent of the total. Because only the free and albumin-bound fractions are generally considered available to tissues, free testosterone can matter clinically even when the total looks normal.
When does total testosterone mislead?
SHBG, the main carrier protein, changes with age and health conditions. When SHBG is high (older age, thyroid disease, liver disease, some medications), more testosterone is bound and free testosterone falls even though the total stays in range. When SHBG is low (obesity, diabetes, hypothyroidism, some medications), free testosterone runs higher than the total would suggest. A man can have a normal total testosterone and a low free testosterone, or the reverse, which is why the guideline directs measurement of free testosterone when total is near the lower limit or when SHBG is likely abnormal.
Situations where free testosterone is the more informative number
- Total testosterone at or near the lower end of the reference range
- Obesity, diabetes, or metabolic syndrome, which typically lower SHBG
- Thyroid disease or liver disease, which can raise SHBG
- Older age, where SHBG tends to rise
- Medications known to affect SHBG, including some seizure medicines and glucocorticoids
How is free testosterone measured?
The gold-standard laboratory method is equilibrium dialysis, which physically separates the free fraction. It is accurate but labor-intensive and not routinely offered by commercial labs. In everyday practice, free testosterone is usually calculated from total testosterone, SHBG, and albumin using a validated equation such as the Vermeulen formula, an approach the Endocrine Society endorses. The two methods agree well in most clinical situations, provided the calculated value is used together with SHBG rather than on its own.
One measure to be wary of is the free androgen index, a simple ratio of total testosterone to SHBG. It is not recommended for men because it becomes inaccurate when SHBG is at the extremes, which is exactly the situation where free testosterone matters most.
What the numbers mean
| Test result | What it tells you |
|---|---|
| Total testosterone (fasting, morning) | The screening measure; levels fall through the day, so a morning sample is standard |
| Free testosterone (calculated or measured) | The unbound fraction that matters when total is borderline or SHBG is abnormal |
| SHBG | The carrier protein that determines how much testosterone is bound; needed to interpret free testosterone |
| Bioavailable testosterone | Free plus albumin-bound testosterone, another way labs express what is available to tissues |
What testing alone cannot tell you
A number, low or normal, is not a diagnosis. The Endocrine Society guideline is explicit that testosterone testing should be prompted by persistent signs and symptoms of low testosterone and that treatment decisions are made on symptoms plus labs, not labs alone. Fatigue, low libido, and mood changes overlap with many other causes, which is why at Zest Regen Health hormone testing happens inside a medical evaluation rather than as an at-home number chase.
Why test timing and repetition matter
Testosterone follows a daily rhythm and peaks in the morning, so guidelines call for a morning fasting blood draw, and a low result should be confirmed with a repeat sample before any treatment decision. At the Columbia clinic, testing is scheduled and interpreted with this in mind, and follow-up blood work after a treatment plan begins is part of the program rather than an afterthought.
Read about men's hormone therapy and how testing guides a plan (/services/mens-hormone-therapy/).
Read about the signs of low testosterone in men (/blog/signs-of-low-testosterone/).
Read about how to prepare for a hormone blood test (/blog/how-to-prepare-for-hormone-blood-test/).
Meet the physicians who evaluate hormone concerns at Zest Regen Health (/dr-francisco-ward/).
Find the clinic at 8860 Columbia 100 Parkway, Suite 216, Columbia, MD (/columbia-md/).
Sources & further reading
- Endocrine Society Clinical Practice Guideline: Testosterone Therapy in Men With Hypogonadism (2018)
- Accurate measurement of total and free testosterone levels for the diagnosis of androgen disorders (2022)
- Free testosterone by direct and calculated measurement versus equilibrium dialysis in a clinical population (2013)
- Society for Endocrinology: UK guidelines on the management of male andropause and testosterone
Next step
If low energy, low libido, or other symptoms have you wondering whether hormones are involved, a free 15-minute consultation at the Columbia clinic can help you decide whether testing is worth doing and what it should include. The team explains what each result can and cannot tell you before any lab is ordered.
Talk with our care team about this topic
A member of our clinical team can answer your questions at the Columbia, Maryland clinic. If Men's Hormone Therapy may be appropriate for you, the first step is a consultation.
Care at Zest Regen Health is physician-guided. Meet our physicians, who evaluate patients at the Columbia, Maryland clinic.