The body maintains very tight control over the calcium circulating in the blood at any given time. The equilibrium is maintained by an elegant interplay of calcium absorbed from the intestines, movement of calcium into and out of the bones, and the kidney’s reclamation and excretion of calcium into the urine.
If the serum calcium level falls, the parathyroid glands release PTH into the blood and this signals cells in bone (osteoclasts) to release calcium from the bone surfaces. PTH also signals the kidney to reclaim more calcium before it is excreted in the urine and also stimulates synthesis of the active form of vitamin D.
There are at least three hormones intimately involved in the regulation of the level of calcium in the blood: parathyroid hormone (PTH), calcitonin and calcitriol (1, 25 dihydroxyvitamin D, the active form of vitamin D). 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. This hormonal “feedback loop” is governed by the parathyroid glands and the calcitonin-secreting cells of the thyroid gland by their constant monitoring of the blood calcium levels. 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).