In PTH deficiency (idiopathic or surgical hypoparathyroidism) or an abnormal target tissue response to PTH (pseudohypoparathyroidism), serum calcium falls and serum phosphate rises. In such patients, 1,25(OH)2D levels are usually low, presumably reflecting the lack of stimulation by PTH of 1,25(OH)2D production. The skeletons of patients with idiopathic or surgical hypoparathyroidism are normal except for a slow turnover rate. A number of patients with pseudohypoparathyroidism appear to have osteitis fibrosa, suggesting that the normal or high PTH levels found in such patients are capable of acting on bone but not on the kidney. The distinction between pseudohypoparathyroidism and idiopathic hypoparathyroidism is made on the basis of normal or high PTH levels but deficient renal response (ie, diminished excretion of cAMP or phosphate) in patients with pseudohypoparathyroidism.
The principal therapeutic goal is to restore normocalcemia and normophosphatemia. Standard therapy involves the use of calcitriol and dietary calcium supplements. However, many patients develop hypercalciuria with this regimen, which limits the ability to correct the hypocalcemia. Full-length PTH (rhPTH 1-84, Natpara) has recently been approved for the treatment of hypoparathyroidism and reduces the need for large doses of calcium and calcitriol with less risk of hypercalciuria.