Calcium is essential for neuromuscular excitability, cardiac automaticity, mitotic division, coagulation, muscle contraction, neurotransmitter and hormone secretion and action, and the activity of many enzymes. Parathyroid hormone (PTH) and vitamin D maintain the extracellular calcium concentration within a narrow range. PTH increases intestinal calcium absorption, increases osteoclastic release of calcium and phosphorus from bone, decreases renal clearance of calcium, and enhances formation of 1,25-dihydroxyvitamin D by the kidney. Levels of ionized calcium and magnesium determine the secretion of PTH. Vitamin D augments the effects of PTH and is necessary for calcium absorption from the GI tract. Calcitonin from thyroid parafollicular cells lowers serum calcium in two ways: by inhibiting osteoclast activity and by inhibiting renal reabsorption of calcium and phosphorus.
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