Summary
Calcium is the most abundant mineral in the human body and is essential for building and maintaining strong bones and teeth. A small but precisely regulated fraction circulates in the blood, where it is critical for muscle contraction, nerve impulse transmission, cardiac rhythm, and blood clotting — any deviation from the normal range can cause serious symptoms.
Serum calcium is tightly controlled by parathyroid hormone (PTH), vitamin D, and calcitonin. A normal serum calcium does not guarantee adequate bone calcium — the body will draw calcium from bones to maintain blood levels, so bone density can fall even when serum calcium is normal if dietary intake is insufficient.nnElevated calcium (hypercalcaemia) is most commonly caused by primary hyperparathyroidism or malignancy. Low calcium (hypocalcaemia) is usually caused by vitamin D deficiency, hypoparathyroidism, or magnesium deficiency impairing PTH action.
What It Is
Calcium circulates in the blood in three forms: ionised (free, biologically active — ~50%), protein-bound predominantly to albumin (~40%), and complexed to anions (~10%). Standard laboratory tests measure total serum calcium. Corrected calcium adjusts for albumin concentration, which is important because low albumin (as in malnutrition or liver disease) makes total calcium appear falsely low while ionised calcium is normal.nnCalcium homeostasis is primarily regulated by: PTH (released when calcium falls — it stimulates bone resorption, renal calcium reabsorption, and vitamin D activation), vitamin D (promotes intestinal calcium absorption), and calcitonin (released when calcium rises — inhibits bone resorption).nnChronically low calcium intake leads to secondary hyperparathyroidism, progressive bone density loss, and ultimately osteoporosis, even when serum calcium is maintained within range.
Functions
Bone and teeth structure
99% of body calcium is stored in the skeleton as hydroxyapatite — providing structural rigidity and serving as a calcium reserve for blood homeostasis.
Muscle contraction
Calcium release from the sarcoplasmic reticulum triggers the actin-myosin interaction that underlies every skeletal and cardiac muscle contraction.
Nerve impulse transmission
Calcium influx through voltage-gated channels at synapses triggers neurotransmitter release, enabling communication between nerve cells.
Cardiac rhythm and coagulation
Regulates the electrical activity of the heart (pacemaker cells are calcium-dependent) and is an essential cofactor in the coagulation cascade.
Reference Ranges
Serum Corrected Calcium
Measured in mmol/L| Status | Range (mmol/L) | What it means |
|---|---|---|
| Low | < 2.15 | Hypocalcaemia — risk of tetany, muscle cramps, cardiac arrhythmia, and seizures in severe cases. |
| Borderline | 2.15–2.25 | Low-normal — assess vitamin D, PTH, and magnesium. Consider bone density if sustained. |
| Normal | 2.25–2.55 | Tightly regulated range for normal neuromuscular, cardiac, and bone function. |
| Elevated | > 2.55 | Hypercalcaemia — investigate for hyperparathyroidism, malignancy, or excess vitamin D supplementation. |
Always interpret as corrected (albumin-adjusted) calcium, not total calcium. Borderline or abnormal results require measurement of PTH, vitamin D, and urinary calcium to identify the underlying cause.
Symptoms of Imbalance
Calcium symptoms depend on whether levels are too low (hypocalcaemia) or too high (hypercalcaemia).
- Muscle cramps and tetany — painful involuntary contractions
- Tingling and numbness around the mouth, hands, and feet
- Brittle nails and dry skin
- Dental problems and enamel defects
- Seizures (in severe acute hypocalcaemia)
- Muscle spasm producing positive Trousseau's and Chvostek's signs
- Anxiety and cognitive impairment in chronic deficiency
- Fatigue, weakness, and generalised malaise
- Nausea, vomiting, and loss of appetite
- Excessive thirst and frequent urination
- Constipation
- Confusion, cognitive decline, and depression
- Kidney stones (calcium oxalate — common in hypercalcaemia)
- Bone pain and pathological fractures (in malignancy-related hypercalcaemia)
Causes of Imbalance
- Vitamin D deficiency (reduces intestinal calcium absorption)
- Hypoparathyroidism (surgical or autoimmune)
- Magnesium deficiency (impairs PTH secretion and action)
- Chronic kidney disease (impaired vitamin D activation)
- Malabsorption — coeliac disease, Crohn's disease
- Certain medications (bisphosphonates, chemotherapy, loop diuretics)
- Hypoalbuminaemia (falsely low total calcium — check corrected value)
- Primary hyperparathyroidism (most common cause — usually a parathyroid adenoma)
- Malignancy (bone metastases or PTHrP-secreting tumours)
- Excessive vitamin D supplementation
- Sarcoidosis and other granulomatous diseases
- Thiazide diuretics
- Immobilisation in Paget's disease
- Familial hypocalciuric hypercalcaemia (genetic)
FAQs
A raised serum calcium (hypercalcaemia) most commonly indicates primary hyperparathyroidism — a benign parathyroid adenoma that over-secretes PTH. It can also result from certain cancers (bone metastases or PTHrP-secreting tumours), excess vitamin D supplementation, or sarcoidosis. Hypercalcaemia above 3.0 mmol/L is a medical emergency. Any elevated calcium must be confirmed on a repeat test with concurrent PTH measurement.
About 40% of blood calcium is bound to albumin. If albumin is low (as in malnutrition, liver disease, or critical illness), total calcium can appear falsely low when ionised (active) calcium is normal. The corrected calcium calculation adjusts for albumin: Corrected Ca = measured Ca + 0.02 × (40 – albumin). Corrected calcium gives a more accurate reflection of the physiologically relevant calcium level.
Yes. Good non-dairy calcium sources include fortified plant milks, firm tofu, canned fish with bones (sardines, salmon), almonds, sesame seeds, kale, and broccoli. Calcium absorption from some plant sources is lower due to oxalates (spinach, almonds) or phytates. Adequate vitamin D is essential to absorb dietary calcium effectively — without it, even a high calcium intake will be poorly utilised.
This is a nuanced area. Large studies have suggested that calcium supplements (not dietary calcium) may slightly increase cardiovascular risk, possibly due to peak spikes in serum calcium after supplementation. Current guidance generally recommends prioritising dietary calcium and using supplements only to bridge a gap when diet is insufficient. Taking calcium alongside vitamin K2 (which directs calcium to bones rather than arteries) may mitigate this concern.
The UK recommendation for adults is 700 mg/day, rising to 1000–1200 mg/day in older adults (post-menopausal women and men over 55) at risk of osteoporosis. Pregnant and breastfeeding women need 1000 mg/day. Testing serum calcium reflects acute regulation, not long-term sufficiency — bone density scanning (DEXA) and dietary assessment are more informative for long-term adequacy.
References
- Bilezikian JP, et al. Primary hyperparathyroidism. Nat Rev Dis Primers. 2018;4:17136. View source
- Peacock M. Calcium metabolism in health and disease. Clin J Am Soc Nephrol. 2010;5(Suppl 1):S23–S30. View source
- Bolland MJ, et al. Effect of calcium supplements on risk of myocardial infarction and cardiovascular events. BMJ. 2010;341:c3691. View source