PTH and vitamin D deficiency: The most common models of vitamin D deficiency using a dietary approach, a vitamin D deficient diet, not only are time consuming and thus costly, but also have major limitations for result interpretation, specifically, due to the abnormalities in serum PTH, Ca, and P levels, which develop from the prolonged time required to achieve vitamin D deficiency due to the half-life of 15–18 days of serum 25D [15, 16].