Through their actions and interactions on bone, kidney and the gastrointestinal (GI) tract, the three calcitropic hormones, parathyroid hormone (PTH), calcitonin (CT), and vitamin D metabolites, especially the 1,25-dihydroxyvitamin D (1,25-D) metabolite, act to maintain serum (and extracellular fluid) calcium within a normal range, a range that optimally subserves many calcium-requiring physiological functions such as neural transmission and muscle contraction. A perturbation in serum calcium, which plays an important role in regulating the concentrations of the calcitropic hormones, will cause a homeostatically appropriate and often reciprocal change in the secretion of PTH by the parathyroid glands, CT by the thyroidal C cells, and in the production of 1,25-D by the kidneys. These responses are designed to return the serum calcium, and, to a lesser extent, the serum phosphorus and magnesium to normal, with the skeleton acting as a reservoir for these minerals that can be emptied or filled.

Thus, the patient with hypocalcemia (nonparathyroid) will have an increased serum PTH and 1,25-D and a decreased serum CT. This will result in increased GI absorption of calcium, increased bone resorption, and decreased renal calcium excretion all acting to increase the serum calcium toward normal. The patient with hypercalcemia (nonparathyroid) will have a decreased serum PTH and 1,25-D and an increased serum CT. This will result in decreased GI absorption of calcium, decreased bone resorption, and increased renal calcium excretion all acting to decrease the serum calcium toward normal. In diseases that involve one of the three calcitropic hormones, the serum concentrations of the other two will change to either amplify the effect of the primary abnormality or to defend against the calcium perturbation. Although these compensatory mechanisms act to restore serum calcium to normal, the homeostasis will not be complete until the primary abnormality has been corrected. In addition to these three calcitropic hormones, other hormones, cytokines, and growth factors play an important role in calcium metabolism. Among the other important hormones are insulin, growth hormone, and the gonadal and adrenal steroids and thyroid hormone (Table 20). They are discussed in other chapters.