Free testosterone (calculated)
What it measures, and what changes the number.
Free testosterone is the portion of testosterone in the blood that is not bound to SHBG or albumin. This version is not measured directly. It is estimated with a formula, most often the Vermeulen equation, from total testosterone, SHBG and albumin.
Because it is derived from those markers, it is always read with them. It is often reported next to free testosterone % and bioavailable testosterone.
The result depends on how accurately total testosterone and SHBG were measured, so the assays used matter. Some labs plug in a fixed albumin value instead of a measured one. Testosterone is highest in the morning, so samples are usually taken early in the day. Spanish reports may print it as Testosterona libre.
Often read alongside: Total testosterone, SHBG, Albumin, Free testosterone %, Bioavailable testosterone