RT Book, Section A1 Arora, Nayan A1 Jefferson, J. Ashley A2 Papadakis, Maxine A. A2 McPhee, Stephen J. A2 Rabow, Michael W. A2 McQuaid, Kenneth R. SR Print(0) ID 1193125085 T1 Disorders of Calcium Concentration T2 Current Medical Diagnosis & Treatment 2023 YR 2023 FD 2023 PB McGraw-Hill Education PP New York, NY SN 9781264687343 LK accessmedicine.mhmedical.com/content.aspx?aid=1193125085 RD 2024/03/29 AB The normal total plasma (or serum) calcium concentration is 8.5–10.5 mg/dL (2.1–2.6 mmol/L). Ionized calcium (normal: 4.6–5.3 mg/dL [1.16–1.31 mmol/L]) is the physiologically active portion and is necessary for muscle contraction and nerve function. In most situations, measuring total calcium concentration is sufficient since changes mirror that seen in ionized calcium concentration; exceptions include patients with hypoalbuminemia and certain acid-base disorders. Calcium homeostasis is primarily maintained by parathyroid hormone (PTH) and calcitriol (1,25-dihydroxyvitamin D). Calcium also autoregulates by binding to calcium sensing receptors (CaSR) in the parathyroid gland to reduce PTH secretion and in the kidney to enhance calcium excretion. Functional defects in this protein are associated with diseases of abnormal calcium metabolism, such as familial hypocalcemia and familial hypocalciuric hypercalcemia (eTable 21–1).