Pseudohypoparathyroidism (all with hypocalcemia andhyperphosphatemia) may occur in one of four forms as follows: 1.
Type Ia: an abnormality of the Gsa protein linking the PTH receptor to adenylate cyclase; biologically active PTH is secreted in great quantities but does not stimulate its receptor 2.
Type Ib: normal phenotype, normal Gsa with abnormalities in the production of adenylate cyclase 3.
Type Ic: abnormal phenotype, normal production of adenylate cyclase, but a distal defect eliminates the effects of PTH 4.
Type II: normal phenotype, normal production of adenylatecyclase, with a postreceptor defect, close to type Ib
Pseudohypoparathyroidism is an autosomal dominant condition that may present at birth or later. Pseudohypoparathyroidism is associated with Albright hereditary osteodystrophywhose clinical manifestations include short stature, stockybody habitus, round facies, short fourth and fifth metacarpals,calcification of the basal ganglia, subcutaneous calcification,and developmental delay. Albright hereditary osteodystrophymay be inherited separately so that a patient may have a normal appearance with hypocalcemia or may have the Albrighthereditary osteodystrophy phenotype with normal serum calcium, phosphate, PTH, and response to PTH.
Transient neonatal hypocalcemia. During the first 3 days after birth, serum calcium concentrations normally decline in response to withdrawal of the maternal calcium supply via the placenta. Sluggish PTH response in a neonate may result in a transient hypocalcemia. Hypocalcemia caused by attenuated PTH release is found in infants of mothers with hyperparathyroidism and hypercalcemia; the latter suppresses fetal PTH release, causing transient hypoparathyroidism in the neonatal period.
25(OH)D, 25-Hydroxyvitamin D; Nl, normal; sl, slight; ↑, high; ↓, low.
Normal serum magnesium concentrations are required for normal parathyroid gland function and action. Hypomagnesemia may cause a secondary hypoparathyroidism, which responds poorly to therapies other than magnesium replacement.
The etiology of hypocalcemia usually can be discerned by combining features of the clinical presentation with determinations of serum ionized calcium, phosphate, alkaline phosphatase, PTH (preferably at a time when the calcium is low), magnesium, and albumin. If the PTH concentration is not appropriately elevated in relation to the low serum calcium, hypoparathyroidism (transient, primary, or caused by hypomagnesemia) is present. Vitamin D stores can be estimated by measuring serum 25-hydroxyvitamin D. Renal function is assessed by a serum creatinine measurement or determination of creatinine clearance (Table 176-1).
Treatment of severe tetany or seizures resulting from hypocalcemia consists of intravenous calcium gluconate (1 to 2 mL/kg of a 10% solution) given slowly over 10 minutes while cardiac status is monitored by electrocardiogram for bradycardia, which can be fatal. Long-term treatment of hypoparathyroidism involves administering vitamin D, preferably as 1,25-dihydroxyvitamin D and calcium. Therapy is adjusted to keep the serum calcium in the lower half of the normal range to avoid episodes of hypercalcemia that might produce nephrocalcinosis and to avoid pancreatitis.
Available @ StudentConsult.com