Harrison's Hypocalcemia Internal Medicine Chapter 47. Hypercalcemia and HYPERCALCEMIA AND HYPOCALCEMIA: INTRODUCTION The calcium ion plays a critical role in normal cellular function and signaling, regulating diverse physiologic processes such as neuromuscular signaling, cardiac contractility, hormone secretion, and blood coagulation. Thus, extracellular calcium concentrations are maintained within an exquisitely narrow range through a series of feedback mechanisms that involve parathyroid hormone (PTH) and the active vitamin D metabolite 1,25-dihydroxyvitmin D [1,25(OH)2D]. These feedback mechanisms are orchestrated by integrating signals between the parathyroid glands, kidney, intestine, and bone (Fig. 47-1) (Chap. 346). Figure 47-1 Feedback mechanisms maintaining extracellular calcium concentrations within a narrow, physiologic range [– mg/dL (– mM)]. . | Chapter 047. Hypercalcemia and Hypocalcemia Part 1 Harrison s Internal Medicine Chapter 47. Hypercalcemia and Hypocalcemia HYPERCALCEMIA AND HYPOCALCEMIA INTRODUCTION The calcium ion plays a critical role in normal cellular function and signaling regulating diverse physiologic processes such as neuromuscular signaling cardiac contractility hormone secretion and blood coagulation. Thus extracellular calcium concentrations are maintained within an exquisitely narrow range through a series of feedback mechanisms that involve parathyroid hormone PTH and the active vitamin D metabolite 1 25-dihydroxyvitmin D 1 25 OH 2D . These feedback mechanisms are orchestrated by integrating signals between the parathyroid glands kidney intestine and bone Fig. 47-1 Chap. 346 . Figure 47-1 Parathyroid glands PTH ECF Ca Kidney Intestine Source fouci as. t rp r CH. sr un ld H u r SL Lonso t L. Jameion JL. Lojc t c J Htmson s of Median 17th Edition http _ rvwv Copyright . Th Mcsriu-Hill Companlaij Inc. All rlg itr riSSiMW . W Bone Feedback mechanisms maintaining extracellular calcium concentrations within a narrow physiologic range mg dL . A decrease in extracellular ECF calcium Ca2 triggers an increase in parathyroid hormone PTH secretion 1 via activation of the calcium sensor receptor on parathyroid cells. PTH in turn results in increased tubular reabsorption of calcium by the kidney 2 and resorption of calcium from bone 2 and also stimulates renal 1 25 OH 2D production 3 . 1 25 OH 2D in turn acts principally on the intestine to increase calcium absorption 4 . Collectively these homeostatic mechanisms serve to restore serum calcium levels to normal. Disorders of serum calcium concentration are relatively common and often serve as a harbinger of underlying disease. This chapter provides a brief summary of the approach to patients with altered serum calcium levels. See Chap. 347 for a detailed discussion of this topic. HYPERCALCEMIA Etiology The .
