The last mechanism examines the relationship between vitamin D and calcium in OA, and regarding its role on calcium metabolism in the body, it points out that affecting by this disease and body vitamin D status is predictable which several epidemiologic studies have shown this relationship.44 Specifically, according to the studies, although vitamin D deficiency causes secondary hyperthyroidism and this lead to an increase in bone turnover and a reduction in BMD, if vitamin D deficiency and high Parathyroid hormone (PTH) occur simultaneously, the possible incidence of OA increases.45 Here, PTH is linked to vitamin D deficiency.