**3. Parathyroid hormone**

Parathyroid hormone is an amino acid hormone composed of 84 amino acids, synthesized in the parathyroid glands. It is initially synthesized as pre-pro-PTH

#### *Prognostic Value of Serum Parathyroid Hormone in Patients with End-Stage Renal Disease DOI: http://dx.doi.org/10.5772/intechopen.100229*

with 115 amino acids that is converted into pro-PTH with 90 amino acids. The latter will be cleaved to form the active form of PTH. The response of parathyroid glands to hypocalcemia by secreting PTH is so powerful. This response occurs within seconds of hypocalcemia in an attempt to rapidly restore normal plasma calcium levels. In addition to de novo synthesis, PTH is stored in secretory granules within the parathyroid glands and both represent the sources of circulatory PTH. The stored PTH within the parathyroid glands explains the rapid release of PTH from the parathyroid glands in response to short term hypocalcemia while prolonged hypocalcemia stimulates synthesis and release of PTH by parathyroid glands [6]. The estimated half-life of PTH is only a few minutes after which it is rapidly eliminated from circulation by the liver and kidneys [7]. The direct action of PTH involves three main organs; bone, kidneys and small intestine. In respect of its effect on the bones, PTH plays a vital role in releasing calcium to the circulation indirectly through the activation of osteoclasts (bone resorption). This action is preceded by stimulating the differentiation of osteoblasts into osteoclasts. This effect of PTH on the bones has a great impact on the rapid correction of plasma calcium levels in short-term hypocalcemia. On the kidneys, PTH has multiple functions that are essential in maintaining plasma calcium levels. At the distal convoluted tubules and collecting ducts, PTH mediates the reabsorption of calcium that has not been reabsorbed at the proximal convoluted tubules. Furthermore, PTH enhances the elimination of phosphate by decreasing the rate of its reabsorption, an action that is indirectly participating in maintaining plasma calcium as less phosphate will be available to bind with plasma free calcium [8]. Another vital action of PTH on the kidney includes increasing the production of 1 alpha-hydroxylase in the proximal convoluted tubules. The final step of activation of vitamin D is catalyzed by this enzyme. This active form of vitamin D mediates the absorption of calcium by the intestine in both transcellular and paracellular pathways in addition to its role in preventing loss of calcium in urine by enhancing its reabsorption at the distal convoluted tubules [9]. Currently, PTH is measured using the second generation intact PTH assay which has the ability to detect various PTH fragments including fulllength (1–84) PTH and long C-PTH fragments, primarily (7–84) PTH, although the differences in the effects of each fragment on various systems in the body are not clear yet [10].
