The term pseudohypoparathyroidism (PHP) designates a heterogeneous group of rare diseases in which renal resistance to parathyroid hormone (PTH) is the cardinal feature (1). These syndromes are characterized by the presence of hypocalcemia, hyperphosphatemia and low plasma concentrations of calcitriol in patients with a normal renal function, findings also observed in hypoparathyroidism. In PHP, however, these abnormalities are present in spite of high concentrations of circulating immunoreactive PTH, clearly demonstrating that the pathogenesis of hypocalcemia in PHP and hypoparathyroidism is different. A variety of abnormalities that are not seen in hypoparathyroidism can also be present in some, but not all, patients with PHP, including Albright's osteodystrophy, resistance to other peptide hormones and increased bone resorption. This clinical and biological heterogeneity of PHP is explained by the existence of numerous different molecular defects that can result in resistance to the actions of PTH.
Parathyroid hormone acts on its target cells through