The parathyroid glands are the part of the endocrine system responsible for the regulation of calcium in the human body. There are four parathyroid glands, all about the size of a grain of rice, located on the posterior of the thyroid gland. The parathyroid glands control the levels of calcium by secreting parathyroid hormone, which stimulates the removal of calcium from the bones of the skeletal system, where it is principally stored. Calcium, in addition to being important for strong bones and strong teeth, is also instrumental in regulating heartbeat, blood pressure, skeletal muscle contraction, and maintaining flexibility in connective tissues, such as ligaments and tendons. Calcium may be obtained from foods in the diet, but calcium supplements are recommended to help maintain healthy levels of calcium in the body. Calcium accounts for about two percent of total body weight, most of which is found in the teeth and the bones.
Calcium occurs in the body in three major locations. Calcium is stored within the matrix of the bones, as a component of various molecules within cellular structures, and as a cation dissolved in blood and extracellular fluids. It is vital that the dissolved calcium stay within limits; either too much calcium or too little can result in a variety of problems affecting the heart and nervous system.
When blood calcium levels begin to fall, the parathyroid glands secrete parathyroid hormone (PTH). PTH stimulates osteoclasts to release calcium from the bone matrix. PTH also stimulates the kidneys to conserve calcium, and also to produce calcitriol, the active form of Vitamin D, which causes the small intestine to increase its uptake of calcium.
Rising levels of blood calcium cause the thyroid to produce the hormone calcitonin, which slows the production of PTH, inhibits the activity of osteoclasts, and increases the amount of calcium excreted in the urine.