In healthy, nonpregnant, unstarved adults, the role of calcitonin is unclear. Learning Objectives Explain how blood calcium levels are regulated by parathyroid hormone. Key Points The parathyroid hormone PTH , secreted by the parathyroid glands, is responsible for regulating blood calcium levels; it is released whenever blood calcium levels are low.
PTH increases blood calcium levels by stimulating osteoclasts, which break down bone to release calcium into the blood stream. PTH increases blood calcium levels by increasing the amount of calcium resorbed by the kidneys before it can be excreted in the urine. PTH increases blood calcium levels by triggering the formation of calcitriol, which increases absorption of dietary calcium through the intestines.
Calcitonin, a hormone produced by the thyroid, acts in opposition to PTH by inhibiting osteoclasts, stimulating osteoblasts, and increasing excretion of calcium into the urine by the kidneys. Key Terms osteoblast : a mononucleate cell from which bone develops parathyroid hormone : a polypeptide hormone that is released by the chief cells of the parathyroid glands and is involved in raising the levels of calcium ions in the blood calcitonin : a hormone, secreted by parenchymal cells, that regulates calcium and phosphate metabolism hypoparathyroidism : deficiency of parathyroid hormone hyperparathyroidism : an abnormal increase in parathyroid gland activity calcitriol : the active metabolite 1,dihydroxycholecalciferol of vitamin D3 that is involved in the absorption of calcium osteoclast : a large multinuclear cell associated with the resorption of bone.
Hormonal Control of Blood Calcium Levels Regulation of blood calcium concentrations is important for generation of muscle contractions and nerve impulses, which are electrically stimulated. It increases blood calcium levels by stimulating the resorption of bones, increasing calcium resorption in the kidneys, and indirectly increasing calcium absorption in the intestines.
PTH comes from the parathyroid glands located behind the thyroid gland in the lower part of the neck and calcitonin comes from cells in the thyroid gland, both of which monitor and maintain calcium levels in the blood. The active form of vitamin D is synthesized in the kidney under the control of PTH.
Special cells that reside in the thyroid gland along with thyroid hormone containing cells release another hormone, calcitonin, into the blood. Calcitonin signals osteoclasts to slow down removal of calcium from bone; this action tends to lower levels of blood calcium.
Conversely, shutting off calcitonin allows osteoclasts to get back in business to release needed calcium from bone. The PTH system provides long-term, day-to-day regulation of calcium levels by many hormones working in concert.
The body also has a minute to minute regulation of calcium levels from osteocytes in bone—these cells can instantly release needed calcium or instantly stop releasing calcium depending on immediate needs too little or too much calcium coming into the bloodstream. Share on facebook. Share on twitter. Share on linkedin. Share on email. Share on print. How the Body Maintains Calcium Levels. A primary problem in the parathyroid glands, producing too much parathyroid hormone causes raised calcium levels in the blood hypercalcaemia and this is referred to as primary hyperparathyroidism.
There is a similar but much rarer condition called tertiary hyperparathyroidism that causes hypercalcaemia due to excess parathyroid hormone production on the back drop of all four glands being overactive. Secondary hyperparathyroidism occurs in response to low blood calcium levels and is caused by other mechanisms, for example, kidney disease and vitamin D deficiency. Mild primary hyperparathyroidism often causes few if any symptoms and is frequently diagnosed by finding a high calcium concentration on a routine blood test.
Treatment may be by surgical removal of the affected gland s parathyroidectomy. Further information on the symptoms for each condition can be found in the individual articles. Too little parathyroid hormone or hypoparathyroidism , is a rare medical condition. It can result in low levels of calcium in the blood hypocalcaemia. It is usually treated medically with oral calcium and vitamin D analogues but the availability of parathyroid hormone replacement therapy may change the approach to treatment for some patients.
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