In Vitamin D deficiency rickets, which vitamin D metabolite is typically decreased?

• A. 1,25(OH)2 vitamin D
• B. Calcium
• C. Phosphorus
• D. 25-hydroxy vitamin D

Answer: (D)

25-hydroxy vitamin D is the circulating form that best reflects overall vitamin D status because it represents the stored vitamin D produced in the liver and accumulated from sun exposure and diet. In deficiency rickets, these stores are depleted, leading to a reduced 25(OH)D level. The active metabolite, 1,25-dihydroxy vitamin D, depends on kidney function and parathyroid hormone activity and can be normal or variably decreased or even increased early on, so it isn’t a reliable marker of deficiency. Calcium and phosphorus are minerals affected by the deficiency through secondary hyperparathyroidism, but they are not vitamin D metabolites. Thus, the metabolite most typically decreased is 25-hydroxy vitamin D.