Definition of Pseudohypoparathyroidism:

Pseudohypoparathyroidism is a genetic disorder that resembles hypoparathyroidism (lowered levels of parathyroid hormone), but is caused by a lack of response to parathyroid hormone rather than having too little of the hormone itself.

Causes, incidence, and risk factors:

Parathyroid hormone is a hormone produced by the parathyroid glands that helps regulate calcium and phosphate 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 enough PTH, but the body cannot respond to it. The body is "resistant" to the effects of PTH. As a result, pseudohypoparathyroidism is very similar to hypoparathyroidism, with low calcium levels and high phosphate levels in the blood. 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 only in the kidneys. As a result, the calcium and phosphate problems are seen, but not the rest of the syndrome. Type Ib is less understood than type Ia. Type II is very similar to type I in its clinical features, but the events that take place in the kidneys are 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.