Figure 1217

Pseudohypoparathyroidism applies to a heterogeneous group of hereditary disorders whose common feature is resistance to parathyroid hormone (PTH). Affected patients are hypocalcemic and hyperphosphatemia despite elevated plasma PTH levels. Hypocalcemia and hyperphophatemia result from the combined effects of defective PTH-mediated calcium reabsorption in the distal convoluted tubule and reduced formation of 1,25-dihydroxy-vitamin D3. The latter leads to defects in renal phosphate excretion, calcium mobilization from bone, and gastrointestinal calcium reabsorption. Differences in clinical features and urinary cyclic adenosine monophosphate response to infused PTH provide the basis for distinguishing three distinct subtypes of pseudohypoparathyroidism (type Ia, type Ib, and type II) [26].

Pseudohypoparathyroidism type Ia (Albright's hereditary osteo-dystrophy) is associated with a myriad of physical abnormalities and resistance to multiple adenylate cyclase-coupled hormones, most notably thyrotropin and gonadotropin [27]. The molecular defect in a guanine nucleotide-binding protein (Gs) blocks the coupling of PTH and other hormone receptors to adenylate cyclase. The molecular defect has not been identified in type Ib, although specific resistance to PTH suggests a defect in the PTH receptor. Oral supplementation with 1,25 dihydroxy-vitamin D3 and, if necessary, oral calcium, is used to correct the hypocalcemia and minimize PTH-induced bone disease [26]. Pseudohypoparathroid-ism type II may be an acquired disease.

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