Parathormone is stored in intracellular vesicles after its synthesis. Secretion of PTH is regulated by plasma Calcium concentration. Fall in plasma calcium induces parathormone release and rise inhibits secretion.
Changes in phosphate concentration in plasma affect parathormone secretion indirectly by altering Calcium concentration.
Parthyroid Hormone increases plasma calcium levels by increasing resorption of calcium from bone and decreasing the excretion of calcium in urine. It also promotes phosphate excretion which tends to supplement the hypercalcaemic effect.
Mechanism of action
The parathormone receptor is a G protein coupled receptor which on activation increases cAMP formation and intracellular Ca2+ in target cells. In bone the target cell appears to be the osteoblast because parathormone receptors are not expressed on the surface of osteoclasts.
It has been proposed that parathormone osteoblast complex somehow increases the activity of osteoclasts.
Hyperparathroidism
With increased levels of parathyroid hormone low plasma calcium level may result. This may lead to following
- Low plasma calcium levels
- Tetany
- Convulsions
- Laryngospasm
- Paresthesias
- Cataract
- Psychiatric changes.
Symptoms of hyperparathyriodism are Hypercalcaemia, decalcification of bone – deformities and fractures, metastatic calcification, renal stones, muscle weakness, constipation and anorexia.
Treatment is surgical removal of the parathyroid tumour.
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