PTH and vitamin D deficiency: Thus, PTH levels are related to vitamin D status as well: PTH upregulates 1-α-hydroxylase, the enzyme responsible for 1-α-hydroxylation of 25(OH)D, converting 25(OH)D to 1,25(OH)2D. In individuals who have vitamin D deficiency, the body increases production of PTH to maintain 1,25(OH)2D and calcium homeostasis.