(N/A) In humans,four parathyroid glands are present on the back side of the thyroid gland,one pair each in the two lobes of the thyroid gland.
The parathyroid glands secrete a peptide hormone called parathyroid hormone $(PTH)$.
The secretion of $PTH$ is regulated by the circulating levels of $Ca^{++}$ ions in the blood.
$PTH$ increases the $Ca^{++}$ levels in the blood.
$PTH$ acts on bones and stimulates the process of bone resorption (dissolution/demineralisation).
$PTH$ also stimulates the reabsorption of $Ca^{++}$ by the renal tubules and increases $Ca^{++}$ absorption from the digested food.
Thus,$PTH$ is a hypercalcemic hormone,i.e.,it increases the blood $Ca^{++}$ levels. Along with $TCT$ (Thyrocalcitonin),it plays a significant role in calcium balance in the body.