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:

Effects on serum calcium (raising)
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 is 9.4mg/dl(8-11) or 2.4mmol/L.

Effects on serum phosphate (decrease, with compensation)
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.

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.

Differential Diagnosis
Increased
 * Age
 * Primary hyperparathyroidism
 * Familial hypocalciuric hypercalcemia
 * Familial hyperparathyroidism
 * Gland hyperplasia
 * Lithium therapy
 * Parathyroid malignancy

Decreased
 * Osteomalacia
 * Pseudohyperparathyroidism
 * Ricketts
 * Secondary hyperprathyroidism
 * Vitamin D deficiency
 * Cancer
 * Granulomatous
 * PTH <2-20pg/ml and calcium <2.1mmol/l:Hypoparathyroidism
 * PTH <2-20pg/ml and calcium >2.6mmol/l:hypercalcemia due to tumor

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 (e.g. 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.