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.
Parathyroid 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.