Special Precautions/Comments:

Additional Information:

Calcium is an essential mineral, and the majority (99%) is found within the bone as hydroxyapatite. The remaining 1% is found in the serum and intracellularly. Calcium has a variety of functions including bone growth and remodelling, muscle contraction, blood coagulation, exocytosis and cell signalling. Serum calcium levels are controlled by parathyroid hormone (PTH), calcitonin and vitamin D. 

Calcium circulates in the serum in 3 main forms. Approximately 55% is ionised (the biologically active form), 35% is bound to albumin and 10% is complexed (e.g. calcium phosphate, calcium citrate). The laboratory measures all 3 forms (total calcium) and is adjusted for the albumin concentration to estimate the ‘free’ calcium. The ionised calcium can be measured separately on a blood gas machine.

Hypercalcaemia can arise through increased gastrointestinal absorption, increased bone resorption, decreased bone mineralisation or decreased renal excretion. The most common causes of hypercalcaemia are primary hyperparathyroidism and malignancy. Other causes include vitamin D intoxication, hyperthyroidism (via increased bone turnover) and familial hypocalciuric hypercalcaemia (FHH). 

Hypocalcaemia can arise through decreased gastrointestinal absorption, decreased bone resorption, increased bone mineralisation or increased renal excretion. Causes of hypocalcaemia include hypoparathyroidism, magnesium deficiency (required for PTH release), chronic kidney disease (reduced 1-alpha hydroxylation of vitamin D) and vitamin D dependent rickets type 1 and 2. It can also be seen in acute pancreatitis and sepsis.