Free testosterone %
What it measures, and what changes the number.
This value shows what share of total testosterone circulates unbound, rather than attached to SHBG or albumin. It is expressed as a percent of total testosterone, not as a concentration.
It is read together with total testosterone, SHBG and albumin, because the free fraction depends mostly on how much binding protein is present. It is often shown next to the free testosterone concentration and bioavailable testosterone.
Most labs calculate the fraction from total testosterone, SHBG and sometimes albumin. Specialised labs measure it by equilibrium dialysis. Calculated and measured values are not directly interchangeable. Testosterone follows a daily rhythm and is highest in the morning, so samples are commonly drawn early in the day. Spanish reports may print it as % Testosterona libre.
Often read alongside: Total testosterone, SHBG, Free testosterone (calculated), Albumin, Bioavailable testosterone