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Hypoparathyroidism
Hypoparathyroidism is a condition in which the levels of circulating parathyroid hormone are low. Various causes of hypoparathyroidism exist.
- Autoimmune
- Surgical removal of the parathyroid glands
- Haemochromatosis
- Di George syndrome
- Hypomagnesaemia
Haemochromatosis can cause hypoparathyroidism by infiltration of the glands by the excess, free iron and the ensuing fibrotic response to the iron.
The parathyroid glands can become replaced by the amyloid protein in amyloidosis.
Di George syndrome is a rare congenital disease in which the parathyroid glands, along with the thymus, do not develop.
Magnesium ions are necessary for the parathyroid gland to secrete parathyroid hormone.
Hypoparathyroidism presents with the features of
hypocalcaemia.
The treatment is as for hypocalcaemia.
Pseudohypoparathyroidism
In pseudohypoparathyroidism the patient is insensitive to parathyroid hormone. Thus, the pathological effects are largely as if the patient had hypoparathyroidism but blood tests disclose that the levels of parathyroid hormone in the blood are actually high. There is hypocalcaemia and elevated blood phosphate levels.
Three types of pseudohypoparathyroidism have been described.
Type Ia displays not only hypocalcaemia and high PTH levels but also the features of Albright's hereditary osteodystrophy in which the patient is short, has short fourth and fifth metacarpals, is obese and possibly has mild mental retardation.
Type 1b exhibits only the alterations to blood calcium and phosphate and does not manifest the features of Albright's hereditary osteodystrophy.
Types 1a and 1b are grouped together because they share the same mechanism. The receptor for parathyroid hormone employs an adenylate cyclase coupled G protein second messenger system and it is this G protein that is defective in types Ia and Ib pseudohypoparathyroidism. One consequence of this is that if the patient is given exogenous parathyroid hormone, no rise in cAMP levels in the urine can be detected (as opposed to the normal response in which cAMP levels should increase).
Type II has similar clinical features to type 1b in that only the abnormalities in blood calcium and phosphate are present. However, the underlying biochemical malfunction is different such that cyclic AMP levels are increased in response to parathyroid hormone.
Pseudopseudohypoparathyroidism
The recursively named entity of pseudopseudohypoparathyroidism pertains to patients who have the typical appearance of somebody who has type 1a pseudohypoparathyroidism but who have normal parathyroid hormone biochemistry and therefore have normal blood calcium and phosphate levels.
The cause of pseudopseudohypoparathyroidism may actually be the same G protein defect as in pseudohypoparathyroidism but for some reason the problems with calcium and phosphate metabolism do not occur (this may reflect the phenomenon of imprinting whereby different genes, or parts of genes, are methylated differently depending on whether the gene was inherited from the mother or the father).