PARATHYROID HORMONE

Contents

Functions

Effects on serum calcium (raising)

Effects on serum phosphate (decrease, with compensation)

Feedback regulation

Syndromes

Measurements

References

See also

Functions

Effects on serum calcium (raising)

Parathyroid hormone (PTH), or parathormone, 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 parafollicular cells of the thyroid gland) acts to decrease calcium concentration.
PTH acts to increase the concentration of calcium in the blood by acting upon parathyroid hormone receptor in three parts of the body:^[1]

'Region'	'Effect'
bones	It enhances the release of calcium from the large reservoir contained in the bones.[2] 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.
kidney	It enhances active reabsorption of calcium from distal tubules[3] and the thick ascending limb.
intestine	It 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 (as Ca2+ ions) by the intestine via calbindin.

Effects on serum phosphate (decrease, with compensation)

PTH reduces the uptake of phosphate in the proximal tubules of the kidney.^[4] which means more phosphate is excreted through the urine.
However, PTH also enhances the uptake of phosphate from the intestine and bones into the blood. Slightly more calcium than phosphate is released from the breakdown of bone, and the intestinal absorption of phosphate (mediated by an increase in activated vitamin D) is not as dependent on vitamin D as is that of calcium. The end result is a small net drop in the serum concentration of phosphate.

Feedback regulation

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.^[1]

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.

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.

References

1.
2. Parathyroid hormone - a bone anabolic and catabolic agent., Poole K, Reeve J, , , Curr Opin Pharmacol, 2005
3. http://sprojects.mmi.mcgill.ca/nephrology/presentation/presentation5.htm
4.
5.

See also

★ Calcium metabolism

★ Disorders of calcium metabolism

★ Parathyroid hormone-related protein