The parathyroid glands are tiny, pea-sized glands embedded in the thyroid lobes, usually two glands in each lobe. The parathyroids secrete parathormone (PTH), which regulates the concentrations of calcium (Ca2+) and phosphate (HP02-4) ions in the blood.
When the calcium concentration in the blood bathing the parathyroid glands is low, PTH secretion increases and has the following effects: It stimulates bone cells to break down bone tissue and release calcium ions into the blood. It also enhances calcium absorption from the small intestine into the blood. Finally, PTH promotes calcium reabsorption by the kidney tubules to decrease the amount of calcium excreted in the urine.