The Full Form of PTH is Parathyroid hormone.

Parathyroid hormone is secreted from four parathyroid glands, which are small glands in the neck, located behind the thyroid gland. Parathyroid hormone regulates calcium levels in the blood, largely by increasing the levels when they are too low. It does this through its actions on the kidneys, bones and intestine:

1. Bones – parathyroid hormone stimulates the release of calcium from large calcium stores in the bones into the bloodstream. This increases bone destruction and decreases the formation of new bone.
    
2. Kidneys – parathyroid hormone reduces loss of calcium in urine. Parathyroid hormone also stimulates the production of active vitamin D in the kidneys.
    
3. Intestine – parathyroid hormone indirectly increases calcium absorption from food in the intestine, via its effects on vitamin D metabolism.