Calcium and vitamin D

Key examples

• Calcium carbonate
• Calcium gluconate
• Colecalciferol
• Alfacalcidol

 

Common indications

1. Calcium and vitamin D are used in osteoporosis to ensure positive calcium balance when dietary intake and/or sunlight exposure are insufficient. Other treatments, such as bisphosphonates, may be given to reduce the risk of fragility fractures.
2. Calcium and vitamin D are used in chronic kidney disease to treat and prevent secondary hyperparathyroidism and renal osteodystrophy.
3. Calcium (as calcium gluconate) is used in severe hyperkalaemia to prevent life-threatening arrhythmias. Other treatments, e.g. insulin with glucose, are given to lower the potassium concentration.
4. Calcium is used in hypocalcaemia that is symptomatic (e.g. paraesthesia, tetany, seizures) or severe (<1.9 mmol/L).
5. Vitamin D is used in the prevention and treatment of vitamin D deficiency, including for rickets (in children) and osteomalacia (adults).

 

Mechanisms of action

• Calcium is essential for normal function of muscle, nerves, bone and clotting. Calcium homeostasis is controlled by parathyroid hormone and vitamin D, which increase serum calcium levels and bone mineralisation, and calcitonin which reduces serum calcium levels. In osteoporosis there is a loss of bone mass which increases the risk of fracture. Restoring positive calcium balance either by dietary means or by administering calcium and vitamin D may reduce the rate of bone loss; whether this prevents fractures is less clear. In severe chronic kidney disease, impaired phosphate excretion and reduced activation of vitamin D cause hyperphosphataemia and hypocalcaemia. This stimulates secondary hyperparathyroidism, which leads to a range of bone changes called renal osteodystrophy. Treatment may include oral calcium supplements to bind phosphate in the gut, and alfacalcidol to provide vitamin D that does not depend on renal activation. In hyperkalaemia, calcium raises the myocardial threshold potential, reducing excitability and the risk of arrhythmias. It has no effect on the serum potassium level. The rationale for the use of calcium in hypocalcaemia and vitamin D in vitamin D deficiency is self-explanatory.

 

Important adverse effects

• Oral calcium is usually well tolerated, but may cause dyspepsia and constipation. When administered IV for the treatment of hyperkalaemia, calcium gluconate can cause cardiovascular collapse if administered too fast, and local tissue damage if accidentally given into subcutaneous tissue.

 

Warnings

• Calcium and vitamin D should be avoided in hypercalcaemia.

 

Important interactions

• Oral calcium reduces the absorption of many drugs including iron, bisphosphonates, tetracyclines and levothyroxine. Administered IV, calcium must not be allowed to mix with sodium bicarbonate due to the risk of precipitation.