# Free testosterone

Free testosterone is the small slice of your testosterone that floats free in your blood. Most of it is bound to two proteins, SHBG and albumin. Only the 1 to 4% that is unbound is active right away. The gold-standard test is equilibrium dialysis plus LC-MS/MS. A good shortcut is 'calculated free testosterone' (cFT). You work it out from total testosterone, SHBG, and albumin, using the Vermeulen equation. (Cheaper 'analogue' blood tests are inaccurate. The Endocrine Society advises against them.) Per its 2018 guideline, a calculated value below about 65 pg/mL (220 pmol/L), in a man with symptoms, points to low testosterone. The normal floor in healthy young men sits around 70 pg/mL. Watch the confounders. Anything that shifts SHBG can skew the result: insulin resistance, thyroid disease, oral estrogens. So can the time of day, an acute illness, or differences between labs.

## Sources

- Vermeulen A, Verdonck L, Kaufman JM. (1999). A critical evaluation of simple methods for the estimation of free testosterone in serum. Journal of Clinical Endocrinology & Metabolism. https://doi.org/10.1210/jcem.84.10.6079
- Bhasin S, Brito JP, Cunningham GR, et al.. (2018). Testosterone Therapy in Men With Hypogonadism: An Endocrine Society Clinical Practice Guideline. Journal of Clinical Endocrinology & Metabolism. https://doi.org/10.1210/jc.2018-00229

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