Parathyroid hormone

Functions

Effects on serum calcium (raising)

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 via kidney	It enhances the absorption of calcium in the intestine by increasing the production of activated vitamin D. Vitamin D activation occurs in the kidney. PTH up-regulates 25-Hydroxyvitamin D3 1-alpha-hydroxylase, the enzyme responsible for 1-alpha hydroxylation of 25-hydroxy vitamin D, converting vitamin D to its active form (1,25-dihydroxy vitamin D). This activated form of vitamin D affects the absorption of calcium (as Ca²⁺ ions) by the intestine via calbindin.

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^[4] ^[5] .

Effects on serum phosphate (decrease, with compensation)

PTH reduces the reabsorption of phosphate from the proximal tubule of the kidney^[3] 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.^[6]

Syndromes

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.