We estimated 1,25(OH)2D:24,25(OH)2D VMR of ≥35 to be the threshold value for vitamin D insufficiency, and ≥51 to be predictive of vitamin D deficiency. Our three-dimensional model provides mechanistic insight into the vitamin D-PTH endocrine system, and further substantiates the role of 24,25(OH)2D in human physiology. Here, PTH is linked to vitamin D deficiency.