Abstract: Maintaining normal calcium levels within the body (8.5-10 mg dL-1) requires the action of two hormones in particular: Parathyroid Hormone (PTH) and calcitonin (http://www.bloodbook.com/ranges.html). In lower calcium levels, PTH is released and works in such a way as to increase the calcium back to the normal range. Calcitonin acts exactly in the inverse way by targeting osteoclasts and osteoblasts. A somewhat constant amount of calcium is lost from the body through fecal excretion. In the gut, absorption and secretion of calcium and phosphate occurs, depending on the free ionized calcium in the extracellular fluid. The amount of calcium in the extracellular fluid also influences excretion of calcium in the renal system. The largest pool of calcium is found in bone which is essential in calcium homeostasis. This is because through bone remodelling, calcium may be taken up from the extracellular fluid or given up to the extracellular fluid depending on the presence of hormones in a process known as osteolytic osteolysis. The processes mentioned before are mediated through PTH, calcitonin and 1,25-dihydroxycholecalciferol.
Stephanie Galea and Renald Blundell, 2011. Parathyroid Hormone and Calcitonin Regulating Calcium Levels. Research Journal of Biological Sciences, 6: 183-186.