Pseudohypoparathyroidism

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Overview, Causes, & Risk Factors

Parathyroid hormone (PTH) is a hormone produced by the parathyroid glands that helps regulate calcium and phophate levels in the blood. The effects of PTH are seen in several body systems including the skeletal, gastrointestinal, renal (kidney), muscular, and central nervous system.

In pseudohypoparathyroidism, there is an adequate amount of PTH but the body cannot respond to it. The body is "resistant" to the effects of PTH. As a result, a picture very similar to hypoparathyroidism develops with low calcium levels in the blood and high phophate levels. This results in the characteristic symptoms which are generally first seen in childhood.

There are two different types of pseudohypoparathyroidism, both of which are caused by abnormal genes. Type I can be further divided into two sub-types: type Ia is caused by a one-gene abnormality inherited in an autosomal dominant manner (only one parent needs to have the gene for the child to inherit it). This defect also causes short stature, round face and short hand bones and is also called Albright's hereditary osteodystrophy.

Type Ib is characterized by resistance to PTH confined to the kidney. As a result, the calcium and phosphate problems are seen, but not the rest of the syndrome. The genetic and molecular features of Ib are less clear.

Type II is very similar to type I in its clinical features, but the underlying mechanism in the kidney is different.

When Albright's hereditary osteodystrophy occurs without hypocalcemia (low levels of calcium in the blood), it is known as pseudopseudohypoparathyroidism.

All forms of pseudohypoparathyroidism are very rare.