Chronic vitamin D deficiency causes secondary hyperparathyroidism. Decreases in serum vitamin D levels will result in proportionally higher parathyroid hormone (PTH) levels that serve to maintain normal serum and total body calcium. Vitamin D deficiency reduces intestinal calcium absorption. The decrease in serum calcium levels then triggers PTH release, which quickly corrects the calcium level by taking calcium from bone, increasing renal tubular calcium reabsorption, and increasing renal production of 1,25(OH)2D.
