Vitamin D has long been a familiar concept for any professional working in clinical biology. Nowadays it is becoming known to a large sector of the population. The great research efforts developed over the past decade have led to an explosion in the number of determinations requested of the most qualified metabolite to express the body state of vitamin D, 25-OH vitamin D (25-OHD) free, represents the small fraction not bound to transporter proteins. According to the free hormone hypothesis, it should be considered the best representation of the body state of vitamin D. Unfortunately, limited attention has been paid to this determination since, until recently, the scientific community only had tedious, indirect methods to measure it. A few years ago, a direct measurement method of free 25-OHD was already available to carry out research studies with promising results.