Parathyroid hormone

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Parathyroid hormone
Identifiers
Symbol(s)	PTH
Entrez	5741
OMIM	168450
RefSeq	NM_000315
UniProt	P01270
Other data
Locus	Chr. 11 p15.3-15.1

Parathyroid hormone receptor 1
Identifiers
Symbol(s)	PTHR1 PTHR
Entrez	5745
OMIM	168468
RefSeq	NM_000316
UniProt	Q03431
Other data
Locus	Chr. 3 p22-p21.1

Parathyroid hormone receptor 2
Identifiers
Symbol(s)	PTHR2
Entrez	5746
OMIM	601469
RefSeq	NM_005048
UniProt	P49190
Other data
Locus	Chr. 2 q33

Parathyroid hormone (PTH) is secreted by the parathyroid glands as a polypeptide containing 84 amino acids. It acts to increase the concentration of calcium in the blood, whereas calcitonin (a hormone produced by the thyroid gland) acts to decrease calcium concentration.

1 Functions
2 Syndromes
3 Measurements
4 External links
5 See also

[edit] Functions

[edit] Raising serum calcium

PTH acts to increase the concentration of calcium in the blood in three ways. It enhances the release of calcium from the large reservoir contained in the bones, enhances reabsorption of calcium from renal tubules; and enhances the absorption of calcium in the intestine by increasing the production of vitamin D and upregulating the enzyme responsible for 1-alpha hydroxylation of 25-hydroxy vitamin D, converting vitamin D to its active form (1,25-dihydroxy vitamin D) which effects the actual absorption of calcium by the intestine.

Increased calcium concentration in the blood acts (via feedback inhibition) to decrease PTH secretion by the parathyroid glands. This is achieved by the activation of calcium-sensing receptors located on parathyroid cells.

[edit] Unchanged serum phosphate

PTH leaves the concentration of phosphate in the blood practically unchanged. Although it reduces reabsorption in the proximal tubules of the kidney, it also enhances the uptake of phosphate from the intestine and bones into the blood. These responses to PTH cancel each other out, so the serum concentration of phosphate remains approximately the same.

[edit] Resorbing bone

Bone resorption is the normal destruction of bone by osteoclasts, which are indirectly stimulated by PTH. Stimulation is indirect since osteoclasts do not have a receptor for PTH; rather, PTH binds to osteoblasts, the cells responsible for creating bone. Binding stimulates osteoblasts to increase their expression of RANKL, which can bind to osteoclast precursors containing RANK, a receptor for RANKL. The binding of RANKL to RANK stimulates these precursors to fuse, forming new osteoclasts which ultimately enhances the resorption of bone.

[edit] Syndromes

A high level of PTH in the blood is known as hyperparathyroidism.
- If the cause is in the parathyroid gland it is called primary hyperparathyroidism. The causes are parathyroid adenoma, parathyroid hyperplasia and parathyroid cancer.
- If the cause is outside the gland, it is known as secondary hyperparathyroidism. This can occur in chronic renal failure.

A low level of PTH in the blood is known as hypoparathyroidism. Causes include surgical misadventure (eg inadvertent removal during routine thyroid surgery), autoimmune disorder, and inborn errors of metabolism.

[edit] Measurements

PTH can be measured in the blood in several different forms: intact PTH; N-terminal PTH; mid-molecule PTH, and C-terminal PTH, and different tests are used in different clinical situations.