PTH—See Parathyroid hormone

Parathyroid hormone, a polypeptide of 83 amino acid residues, mol wt 9500, is produced by the parathyroid glands. Release of PTH is activated by a decrease of blood Ca " to below normal levels. PTH increases blood Ca " concentration by increasing resorption of bone, renal reabsorption of calcium, and absorption of calcium from the intestine. A cAMP mechanism is also involved in the action of PTH. Parathyroid hormone induces formation of 1-hydroxylase in the kidney, requited in formation of the active metabolite of vitamin D (see Vitamins, vitamin d). 

Parathyroid hormone also exists in storage vesicles. As much as 80-90% of the proPTH synthesized is degraded before it enters this final storage compartment, especially when Ca + levels are high in the parathyroid cell (see above). PTH is secreted when Ca is low in the parathyroid cells, which contain a several-hour supply of the hormone. 

Osteoblast and osteoclast activity is constantly incorporating Ca into bone and removing it again. There are various hormones that regulate these processes calcitonin increases deposition of Ca in the bone matrix, while parathyroid hormone (PTH) promotes the mobilization of Ca and calcitriol improves mineralization (for details, see p. 342). 

Calcitriol and parathyroid hormone, on the one hand, and calcitonin on the other, ensure a more or less constant level of Ca "" in the blood plasma and in the extracellular space (80-110 mg 2.0-2.6 mM). The peptide parathyroid hormone (PTH 84 AA) and the steroid calcitriol (see p. 374) promote direct or indirect processes that raise the Ca "" level in blood. Calcitriol increases Ca "" resorption in the intestines and kidneys by inducing transporters. Parathyroid hormone supports these processes by stimulating calcitriol biosynthesis in the kidneys (see p. 330). In addition, it directly promotes resorption of Ca "" in the kidneys (see p. 328) and Ca "" release from bone (see B). The PTH antagonist calcitonin (32 AA) counteracts these processes. 

Other target organs for the action of 1,25-dihydroxyvitamin D include the kidneys, bone, muscle,vwand skin. The hormone promotes reabsorption of both Ca2+ and inorganic phosphate by kidney tubules. In bone it binds to a specific receptor where it promotes the mobilization of calcium ions. This effect may result in part from stimulation of calcium-activated ATPase of the outer membrane of bone cells. Dissolution of bone also requires the presence of parathyroid hormone (PTH), the 83-residue hormone secreted by the parathyroid gland. In women past the age of menopause and in elderly men the production of 1,25-dihydroxyvitamin D decreases.w This may be a cause of the serious bone loss (osteoporosis) frequently observed. Treatment with 1,25-dihydroxyvitamin D3 or a synthetic analog seems to be helpful to such individuals. /Xy See also Chapter 30, Section A,5. 

FIGURE 88-3. Effects of vitamin D and parathyroid hormone on calcium balance. See text for details. PTH, parathyroid hormone, Ca, calcium. Pi, phosphorus. 

The peptide hormone parathyrin (PTH), which is produced by the parathyroid gland, stimulates Ca "" resorption in the kidneys and at the same time inhibits the resorption of phosphate. In conjunction with the effects of this hormone in the bones and intestines (see p. 344), this leads to an increase in the plasma level of Ca and a reduction in the level of phosphate ions. 

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