Definition: Free testosterone is the part of testosterone in your blood that is not attached to a protein. Most testosterone is bound to SHBG or albumin. MedlinePlus says free testosterone is more available for the body to use. A free testosterone test measures only this unattached form, while a total test counts everything.
MedlinePlus describes three testosterone tests:
| Test | What it counts |
|---|---|
| Total testosterone | Bound and free testosterone together. The most common test. |
| Free testosterone | Only the unattached form. Less common. |
| Bioavailable testosterone | Free testosterone plus testosterone loosely attached to proteins other than SHBG. |
The Xyosted label puts the free share at about 2%. The Depo-Testosterone label also says about 2%, and adds that the free concentration sets testosterone's half-life.
The 2018 Endocrine Society guideline recommends a fasting morning total testosterone, repeated to confirm, as the first test for low testosterone. It adds free testosterone when total is near the lower limit of normal or when a condition changes SHBG, measured by equilibrium dialysis or estimated with an accurate formula.
On treatment, the guideline says monitoring includes measuring serum testosterone and hematocrit during the first year. Draw timing matters: a result taken near your next shot reads differently from one taken just after. See trough level.
Each lab sets its own reference range, so read your result against the range printed on your own report, and talk with your clinician about what it means for you.
Read more on Dose Track: Low testosterone · Testosterone cypionate guide · Logging lab results in Dose Track