Ionized calcium

Ionized calcium (iCa²⁺), also called free calcium, is the active part of your blood calcium. It makes up about 45 to 50% of the total. The rest is bound to albumin or tied up with anions like phosphate and citrate. Your body holds it in a tight range (1.15 to 1.35 mmol/L) using three hormones, PTH, calcitriol, and calcitonin, acting on your bone, kidney, and gut. Doctors prefer measuring ionized calcium over total calcium when albumin is off, as in critical illness, liver disease, or low albumin. That is because the simple albumin-correction formulas can be unreliable then. High calcium raises suspicion for an overactive parathyroid gland or cancer. Low calcium causes twitchy nerves and muscles and, if severe, abnormal heart rhythms.

Last reviewed: June 22, 2026

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