Parathyroid hormone

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 (Ca²⁺) in the blood, whereas calcitonin (a hormone produced by the parafollicular cells (C 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: PTH half-life is approximately 4 minutes.

Function

Regulation of serum calcium

Parathyroid hormone regulates serum calcium levels through its effects on the following tissues:

PTH was one of the first hormones to be shown to use the G-protein, adenylyl cyclase second messenger system.

Normal total plasma calcium level ranges from 8.5 to 10.2 mg/dL (2.12 mmol/L to 2.55 mmol/L).

Regulation of serum phosphate

PTH reduces the reabsorption of phosphate from the proximal tubule of the kidney which means more phosphate is excreted through the urine.

However, PTH enhances the uptake of phosphate from the intestine and bones into the blood. In the bone, slightly more calcium than phosphate is released from the breakdown of bone. In the intestines, which is mediated by an increase in activated vitamin D, the absorption of phosphate 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.

Vitamin D synthesis

PTH increases the activity of 1-α-hydroxylase enzyme, which converts 25-hydroxycholecalciferol to 1,25-dihydroxycholecalciferol, the active form of vitamin D.

Regulation of PTH secretion

Secretion of parathyroid hormone is chiefly controlled by serum [Ca²⁺] through negative feedback, which is achieved by the activation of calcium-sensing receptors located on parathyroid cells. Calcium sensing receptors work by activating the phospholipase C pathway, presumably through a G_qα type of G protein, which ultimately increases intracellular concentration of calcium, which triggers vesicle fusion and exocytosis of parathyroid hormone. It also inhibits (not stimulates, as some sources state) the cAMP dependent pathway.

Stimulators

Decreased serum [Ca²⁺].
Mild decreases in serum [Mg²⁺].

Inhibitors

Increased serum [Ca²⁺].
Severe decreases in serum [Mg²⁺], which also produces symptoms of hypoparathyroidism (such as hypocalcemia).

Clinical significance

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. In secondary hyperparathyroidism, serum Calcium levels are decreased, which causes the hypersecretion of PTH from the parathyroid glands. PTH acts on the proximal tubules in the kidney to decrease reabsorption of Phosphate (increasing its excretion in urine, decreasing its serum concentration). NOTE: however, in chronic renal failure, because the kidneys are failing they are unable to excrete phosphate in the urine, so in this case of secondary hyperparathyroidism, serum calcium will be decreased, but serum phosphate will be increased.
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.

Measurement

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.

The average PTH level is 10-60 pg/ml.

Translation of "Parathyroid hormone"

Arabic: هرمون الدريقات, Bulgarian: Паратироиден хормон, Czech: Parathormon, Danish: Parathyroideahormon, German: Parathormon, Divehi: ޕަރަތައިރޮއިޑް ހޯރމޯން, Spanish: Parathormona, Basque: Paratohormona, French: Parathormone, Croatian: Paratiroidni hormon, Italian: Paratormone, Hebrew: PTH, Lithuanian: Paratiroidinas, Dutch: Parathormoon, Japanese: パラトルモン, Norwegian (Bokmål): Parathyroideahormon, Polish: Parathormon, Portuguese: Hormônio da paratireoide, Russian: Паратиреоидный гормон, Albanian: Parathormoni, Slovak: Paratyroid, Slovenian: Parathormon, Finnish: Parathormoni, Swedish: Paratyroideahormon, Thai: พาราไทรอยด์ ฮอร์โมน, Turkish: Paratiroid hormon.