PTH and vitamin D deficiency: It is possible that the status of serum 25(OH)D in previous studies mediates the effect of vitamin D supplementation on serum vitamin D. Another study found that vitamin D supplementation was not effective on PTH and concluded that the reason for the lack of PTH suppression can be related to the high prevalence of vitamin D insufficiency at baseline so that vitamin D supplementation could not increase the serum 25(OH)D level to cause PTH suppression.67