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The normal total plasma (or serum) calcium concentration is 8.5–10.5 mg/dL (or 2.1–2.6 mmol/L). Ionized calcium (normal: 4.6–5.3 mg/dL [or 1.15–1.32 mmol/L]) is physiologically active and necessary for muscle contraction and nerve function.

The calcium-sensing receptor, a transmembrane protein that detects the extracellular calcium concentration, is in the parathyroid gland and the kidney. Functional defects in this protein are associated with diseases of abnormal calcium metabolism such as familial hypocalcemia and familial hypocalciuric hypercalcemia (eTable 21–1).

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