 Osteitis fibrosa cystica is the result of unchecked hyperparathyroidism , or the overactivity of the parathyroid glands , which results in an overproduction of parathyroid hormone ( PTH ) . PTH causes the release of calcium from the bones into the blood , and the reabsorption of calcium in the kidney . Thus , excess PTH in hyperparathyroidism causes elevated blood calcium levels , or hypercalcemia . There are four major causes of primary hyperparathyroidism that result in OFC : 
