Almost all the calcium in your body -- more than 99 percent -- is stored in your bones. Although the calcium circulating in your blood comprises less than 1 percent of your total body calcium, maintaining appropriate levels is vital for bone health and normal muscle and nerve function. Three principal hormones regulate blood calcium levels: parathyroid hormone, vitamin D and calcitonin.
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Your blood calcium levels are influenced by regulatory processes that occur in your bones, gut and kidneys. Bones undergo constant remodeling. They are broken down by cells called osteoclasts, which release calcium into the bloodstream. Blood calcium can be taken up by other bone cells, called osteoblasts, which use the mineral to produce new bone. Blood calcium levels are also influenced by how much dietary calcium your small intestine absorbs and how much of the mineral your kidneys excrete.
Located on the back of the thyroid gland in your throat, your parathyroid gland consists of four pea-sized nodules. When blood calcium levels drop, the parathyroid gland secretes parathyroid hormone. This hormone works to raise blood calcium levels by stimulating osteoclasts to break down bone and release calcium into the bloodstream.
PTH also signals your kidneys to conserve calcium by reducing the amount excreted in the urine. The hormone also stimulates kidney production of the active form of vitamin D, which triggers increased calcium absorption from the gut. Low levels of PTH -- due to autoimmune disease or another cause -- can lead to abnormally low blood calcium. High levels of PTH -- most commonly caused by a parathyroid tumor -- cause excessively high levels of blood calcium.
Vitamin D acts as a hormone in your body to help regulate blood calcium. This steroid hormone is so vital in maintaining calcium balance that its active form is sometimes referred to as calcitriol. Vitamin D is necessary for sufficient absorption of dietary calcium in your small intestine. A deficiency in vitamin D can cause low blood calcium, due to impaired calcium absorption. This can lead to weakening of the bones, as seen with the vitamin D deficiency conditions, rickets and osteomalacia. Vitamin D is produced in your skin with sun exposure, and is in some foods or from a supplement.
Calcitonin, which is produced by your thyroid gland, serves to lower blood calcium levels. It counters the actions of parathyroid hormone. Calcitonin inhibits osteoclast function, slowing the breakdown of bone. By opposing the action of parathyroid hormone on the kidneys, it also increases excretion of calcium in the urine. Calcitonin's effects on blood calcium levels are small in comparison to parathyroid hormone's influence. High levels of calcitonin, which may be caused by a thyroid tumor, generally do not result in elevated blood calcium.