What is type Ia pseudohypoparathyroidism?

Updated: Aug 08, 2019
  • Author: Manish Suneja, MD, FASN, FACP; Chief Editor: Vecihi Batuman, MD, FASN  more...
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Answer

Type Ia pseudohypoparathyroidism results from a decrease in the Gs-alpha protein. This disorder comprises the biochemical features of pseudohypoparathyroidism along with the following somatic features of Albright hereditary osteodystrophy (AHO):

  • Short stature
  • Mental retardation
  • Obesity
  • Round-shaped face
  • Brachymetacarpia
  • Brachymetatarsia
  • Subcutaneous bone formation

Laboratory findings in AHO include hypocalcemia, hyperphosphatemia (with normal or high PTH levels), and low calcitriol. Vitamin D may be decreased because of inhibition by elevated levels of phosphorus and by decreased PTH stimulation of 25-hydroxyvitamin D 1-alpha-hydroxylase. The low calcitriol levels, in turn, may cause the resistance to the hypercalcemic effects of PTH in the bone.

The defect of the Gs-alpha protein is not confined to the effects of PTH but also affects other hormonal systems (eg, resistance to glucagon, thyroid-stimulating hormone, gonadotropins). The gene for the Gs-alpha protein is located on chromosome 20. Some family members carry the mutation and display the AHO phenotype but do not have pseudohypoparathyroidism. This is termed pseudo-pseudohypoparathyroidism.


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