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 body’s calcium is found in the skeleton.

Calcium enters the body through the small intestine, moves between plasma and intracellular compartments and bone, and is eventually excreted via the kidney. A complex interplay of three major hormones: parathyroid hormone (PTH), 1,25-dihydroxyvitamin D (calcitriol) and calcitonin regulates these processes.

PTH acts to increase plasma calcium levels by:

• Stimulating release of calcium from bone by direct osteolytic action and via osteoclast upregulation;
• Stimulating reabsorption of calcium in the loop of Henle and distal tubule in the kidney;
• Stimulates enzymatic conversion of 25-hydroxyvitamin D to the active metabolite 1,25-dihydroxyvitamin D (calcitriol) in the kidney. The 1,25-dihydroxyvitamin D in turn increases intestinal absorption of calcium.

PTH also acts to decrease serum phosphate levels.

Calcitonin by contrast inhibits the action of osteoclasts, and inhibits renal reabsorption of calcium, in order to decrease plasama calcium levels.

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.

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.