Parathyroid glands produce parathyroid hormone (PTH) which regulates the circulating level of calcium. It has direct action on bones and kidneys with indirect effects on intestinal tracts through vitamin D metabolism. Increased levels of PTH will result in increased plasma calcium levels. PTH stimulates resorption of calcium and phosphate from bone. It also directly stimulates reabsorption of calcium from the kidney and conversion of vitamin D3 to its biologically active form which in turn enhances absorption of calcium in the intestine. PTH also inhibits absorption of phosphate and bicarbonate in the kidneys.
Simply Put: Parathyroid glands control your body calcium level throgh bone, intestine and kidneys