Ninety nine percent of the body’s calcium is stored in bones. Less than 1 % circulates in the bloodstream. Usual calcium intake is 25 mmol or 1,000 mg/day, mainly in dairy products. Normal corrected total serum calcium in adults is 2.12–2.62 mmol/l. A correction factor is applied because serum calcium is part protein bound and part ionised. In practice this means the laboratory will add 0.02 mmol/l to the measured calcium level for every 1 g/l serum albumin is below 40 g/l.

Serum calcium is regulated by PTH, calcitonin and Vit D. PTH releases calcium from bone, calcitonin does the opposite while 1,25OHD increases absorption of calcium from the gut. Acidosis shifts the equilibrium between protein bound and ionised calcium towards ionised calcium while alkalosis has the opposite effect. This is the reason why both respiratory and metabolic alkalosis may cause symptoms of hypocalcaemia in the presence of a normal total serum calcium.

There is no easy way to classify these. The commonest causes are hypoparathyroidism following parathyroidectomy or damage to parathyroid during total thyroidectomy, vitamin D deficiency, acute and chronic kidney disease. Very low calcium and magnesium may be seen in PPI associated hypocalcaemic hypomagnesaemic hypoparathyroidism. These and other less common causes are summarized in Box 7.1

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