Calcium is the most abundant mineral in the body, essential for bone structure, muscle contraction, nerve signalling and blood clotting. Blood calcium levels are tightly regulated; when they fall outside the normal range, the consequences can range from muscle cramps to cardiac arrhythmia.
Calcium is usually measured as part of a routine metabolic panel or full blood count, so many patients receive a calcium result without having specifically requested it. Understanding what the numbers mean — and why both high and low values matter — helps you have a more informed conversation with your doctor.
Total vs ionised calcium
About 40–45% of blood calcium is bound to albumin (a protein); the rest circulates as ionised (free) calcium, which is the biologically active form. Most routine blood tests measure total calcium.
If your albumin level is low — which is common in malnutrition, liver disease or after surgery — total calcium can appear falsely low. In these cases, doctors calculate 'corrected calcium' using a formula, or order an ionised calcium test directly.
Normal total calcium: 2.1–2.6 mmol/L (8.5–10.5 mg/dL). Normal ionised calcium: 1.15–1.35 mmol/L.
High calcium (hypercalcaemia)
The two most common causes of high calcium are primary hyperparathyroidism (overactive parathyroid glands) and malignancy (some cancers cause the body to release calcium from bones or produce PTH-like hormones).
Mild hypercalcaemia may cause no symptoms. More significant elevations cause 'bones, stones, groans and moans' — bone pain, kidney stones, abdominal discomfort and fatigue or confusion.
- Primary hyperparathyroidism — most common cause; parathyroid gland(s) overproduce PTH
- Malignancy — including multiple myeloma, breast and lung cancer
- Excess vitamin D supplementation
- Sarcoidosis and other granulomatous diseases
- Prolonged immobility
Low calcium (hypocalcaemia)
Low calcium most commonly results from low parathyroid hormone (hypoparathyroidism) — often after thyroid or neck surgery — or from vitamin D deficiency. Symptoms include muscle cramps (tetany), tingling around the mouth and fingertips, and in severe cases, seizures and cardiac arrhythmias.
- Vitamin D deficiency — the most common worldwide cause
- Hypoparathyroidism — after neck surgery or autoimmune
- Renal failure — kidneys can't activate vitamin D
- Acute pancreatitis — calcium is consumed in the inflammatory process
- Low albumin — apparent (not true) hypocalcaemia
What to do with a result outside the normal range
A mildly abnormal calcium result is not an emergency, but it does need investigation. Your doctor will typically repeat the test to confirm, check parathyroid hormone (PTH) and vitamin D levels, and look at kidney function. The treatment depends entirely on the underlying cause — supplementing calcium alone without knowing why it's low rarely fixes the problem and can mask important diagnoses.