Calcium Metabolism

Calcium is the most abundant mineral in the body. It is essential for neurotransmission in muscles and nerves, muscle contraction, cardiac function and blood coagulation. There are 25,000 mmol or approximately 1 kg of calcium in the body. More than 90% of the calcium stores in the body are in the skeleton.

Approximately 40-50% of the calcium in the serum is bound to proteins, primarily albumin, and the remaining calcium is unbound. The unbound portion of calcium in blood is described as ‘free calcium’ or ionised calcium. Total serum calcium assays measure both the unbound and bound portions of calcium. Ionised calcium is responsible for the calcium effects seen clinically in hypercalcaemia. Therefore, ionised calcium is the best measured indicator of active calcium.

Plasma calcium is maintained within the reference range by a complex interplay of three major hormones: parathyroid hormone (PTH), 1,25-dihydroxyvitamin D (calcitriol) and calcitonin. The three hormones act primarily at bone, kidney and small intestine sites to maintain appropriate calcium levels.

Calcium enters the body through the small intestine and eventually is excreted via the kidney. Bone can act as a storage depot. This entire system is controlled through a feedback loop; individual hormones increase or decrease the serum calcium concentration.

Learning bite

For hypercalcaemia to develop, the normal calcium regulation system must be overwhelmed by an excess of PTH, calcitriol, some other serum factor that can mimic these hormones, or a huge calcium load.