PTH and Hypercalcemia: The total calcium balance is regulated by PTH, 1,25(OH)2D3, and calcitonin through their interaction with the kidneys, bones and gastrointestinal tract [10]: a reduction in serum calcium induces PTH and 1,25(OH)2D secretion, increasing both its renal retention and intestinal absorption, whereas hypercalcemia is counterbalanced by calcitonin secretion and PTH inhibition.