PTH and vitamin deficiency: The study indicates that, although the clinical application of free 25OHD measurement still needs to be established, the high correlation found between free and total 25OHD serum concentrations and the absence of significant correlation between free 25OHD and PTH levels suggest that the measurement of free 25OHD in serum does not represent any benefit on that of total 25OHD as marker of vitamin deficiency in healthy paediatric population.