CYP27A1 and vitamin D deficiency: An RCT study showed that the impact of baseline 25(OH)D levels on Alendronate treatment alone indicated that patients with vitamin D deficiency had poorer response to the reduction of bone resorption marker CTX, while patients with sufficient baseline 25(OH)D levels had better response; Moreover, supplementing with vitamin D can not only help control PTH levels, but also enhance the reduction effect of bone resorption markers, especially in patients with high PTH levels or vitamin D deficiency (54).