Figure 1. Physiologic mechanisms of calcium homeostasis. Calcium is raised via the release of parathyroid hormone (PTH). PTH secretion increases Ca2+ absorption in the kidneys and small intestine and decreases excretion through dermal and fecal routes. PTH enhances the formation of calcitriol in the kidneys in concert with insulin-like growth factor 1. Bone resorption is increased. Calcium is lowered via calcitonin secretion from C cells of the parathyroid. Calcitonin downregulates Ca2+ absorption in the kidneys and small intestine, and increases dermal and fecal excretion of Ca2+. Bone resorption is decreased. TRP: transient receptor potential.