PTH and vitamin D deficiency: CaSR plays a critical role in Ca+2 homeostasis. Ca+2 level is maintained by the interplay between the above hormones and the bowel (Ca+2 absorption), kidneys (Ca+2 reabsorption and excretion), and bone (Ca+2 uptake and release). The most common causes of hypocalcemia are PTH and vitamin D deficiencies, while the most common causes of hypercalcemia are primary hyperparathyroidism and malignancies. Hypercalcemia is frequently encountered in malignancies and carries a poor prognosis.