The danger of low levels of calcium in the blood

In our last post, we looked at what happens when there are high levels of calcium in the blood – hypercalcaemia. In this post, we will look at hypocalcaemia, when the blood calcium levels are below the normal level.

An average person should have a blood calcium level with a normal range of 2.1–2.6 mmol/L (8.8–10.7 mg/dL, 4.3–5.2 mEq/L). However, those people with levels less than 2.1 mmol/L can be defined as having hypocalcaemia. Mildly low levels that develop slowly often have no symptoms. Otherwise symptoms may include numbness, muscle spasms, seizures, confusion, or cardiac arrest.

The causes of hypocalcaemia include hypoparathyroidism and vitamin D deficiency. Others causes include kidney failure, pancreatitis, calcium channel blocker overdose, rhabdomyolysis, tumour lysis syndrome, and medications such as bisphosphonates. Diagnosis should generally be confirmed with a corrected calcium or ionized calcium level. Specific changes may be seen on an electrocardiogram (ECG).

Treatment for low calcium levels is easily carried out with an intravenous calcium chloride and possibly magnesium sulphate. Other treatments may include vitamin D, magnesium, and calcium supplements. If due to hypoparathyroidism, hydrochlorothiazide, phosphate binders, and a low salt diet may also be recommended. About 18% of people who are in hospital have hypocalcaemia.

The symptoms of hypocalcaemia can be recalled by the mnemonic “CATs go numb” – Convulsions, Arrhythmias, Tetany, and numbness/parasthesias in the hands and feet and around the mouth.

Hypoparathyroidism is a common cause of hypocalcaemia. Calcium is tightly regulated by the parathyroid hormone (PTH). In response to low calcium levels, PTH induces the kidneys to reabsorb calcium, the kidneys to increase production of calcitriol (the active form of vitamin D) thereby increasing intestinal absorption of calcium, and the bones to release calcium. These actions lead to a re-balance in the blood calcium levels. However, in the setting of absent, decreased, or ineffective PTH hormone, the body loses this regulatory function, and hypocalcaemia ensues.