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Total serum calcium comprises three major forms: ionized calcium (about 50% of total), protein bound (about 40% of total), and calcium complexed with anions such as bicarbonate, citrate, lactate, and phosphate (about 10% of total). Most of the protein-bound calcium is bound to albumin. The ionized, or free, calcium is the metabolically active form of calcium. Results for Ca are expressed as milligrams per deciliter (mg/dl). Ionized calcium concentration can be measured by ion-selective electrodes but this usually is not part of the screening panel of serum chemistry tests. Calcium is absorbed in the intestine with phosphate under the action of vitamin D. Corticosteroids inhibit absorption of calcium. It is stored in the body in bone and excreted through the kidneys. Renal excretion is influenced by PTH, vitamin D, and calcitonin. In horses, renal excretion of calcium is a major mechanism for excreting excess dietary calcium; a much larger percentage of absorbed dietary calcium is excreted by horses than in other species. [Hypercalcemia]
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