Clinical meaning
Calcium is the most abundant mineral in the body, with approximately 99% stored in bones and teeth and only 1% circulating in the blood. Normal total serum calcium ranges from 8.5 to 10.5 mg/dL (2.12-2.62 mmol/L). Approximately 50% of serum calcium is bound to albumin (inactive), 40% is ionized/free (biologically active), and 10% is bound to other anions. The ionized fraction (normal 4.5-5.5 mg/dL or 1.12-1.37 mmol/L) is the physiologically important form that affects neuromuscular excitability, cardiac conduction, blood clotting, and bone metabolism. Three hormones regulate calcium balance: parathyroid hormone (PTH) raises serum calcium by stimulating osteoclast activity (bone resorption), increasing renal calcium reabsorption, and activating vitamin D; calcitonin (from thyroid C-cells) lowers serum calcium by inhibiting osteoclast activity; and active vitamin D (calcitriol) increases intestinal calcium absorption. Calcium and phosphorus have an inverse relationship -- when one rises, the other tends to fall. Calcium also has an important relationship with albumin: low albumin causes falsely low total calcium (correct by adding 0.8 mg/dL for each 1 g/dL decrease in albumin below 4 g/dL). Hypocalcemia (total calcium below 8.5 mg/dL)...