Calcium ion levels in the blood are of fundamental importance to all biological systems. They are involved in e.g. maintenance of action potentials, muscle contraction, cell division, modulation of enzyme activities, secretion etc. Replacement with calcium supplements is the usual treatment of hypoparathyroidism and hypocalcemia.
I. High doses of calcium can cause kidney stones.
II. Oral calcium and vitamin D supplements do NOT reverse the low calcium reabsorption in the kidney, which is typical of hypoparathyroidism.
Extra info: High calcium concentrations in the blood can cause kidney stones. Calcium and vitamin D suppletion indeed elevate the calcium levels in the blood towards normal levels, but these supplements do not normalize calcium reuptake within the kidneys.