Calcium is not just the "bone mineral". In the blood it supports the work of nerves, muscles and the heart and blood clotting, so the body holds its level within very narrow limits.
About 45% of blood calcium is bound to the protein albumin, so when albumin is low the total calcium is "underestimated" — it is recalculated (corrected). The level is regulated by parathyroid hormone and vitamin D, and bone turnover is indirectly reflected by alkaline phosphatase.
Reference ranges
The reference range depends on the lab; typical values for adults:
| Total calcium | 2.15 – 2.55 mmol/L |
| Ionized (active) | 1.15 – 1.30 mmol/L |
| Albumin correction | +0.02 mmol/L for every 1 g/L of albumin below 40 |
| Hypercalcemia | usually hyperparathyroidism or cancer |
| Hypocalcemia | vitamin D deficiency, hypoparathyroidism, CKD |
What an elevated level means
High calcium (hypercalcemia) is caused in 90% of cases by two things: primary hyperparathyroidism (excess parathyroid hormone) and cancer.
Other causes: vitamin D overdose, sarcoidosis, prolonged immobility, certain medications (thiazide diuretics, lithium).
Symptoms: thirst and frequent urination, constipation, nausea, weakness, kidney stones; in severe cases — confusion.
What a low level means
Low calcium (hypocalcemia) is more often linked to vitamin D deficiency, hypoparathyroidism (including after thyroid surgery) and chronic kidney disease.
A separate cause is magnesium deficiency: without it parathyroid hormone works poorly and calcium falls. That is why persistent hypocalcemia is checked together with magnesium.
Symptoms: tingling around the mouth and in the fingers, muscle cramps and spasms, irritability; in severe cases — heart rhythm disturbances.
How the test is done
Calcium is part of the blood biochemistry panel (from a vein), taken fasting. If albumin is low, rely on corrected or ionized calcium — it reflects the active fraction more accurately.
Calcium in isolation is of little use: it is interpreted together with albumin, parathyroid hormone, vitamin D and phosphorus, and for bone questions — with alkaline phosphatase.
What affects the value
- Albumin — low albumin underestimates total calcium (correction or ionized calcium is needed).
- Vitamin D and parathyroid hormone — the main regulators of the level.
- Magnesium deficiency — impairs parathyroid hormone and lowers calcium.
- Medications: thiazide diuretics and lithium (raise), loop diuretics (lower).
- Prolonged immobility — leaches calcium out of the bones.
- Blood stasis during the draw and pH — affect ionized calcium.
Check for calcium deficiency
Assess your risk of hidden calcium deficiency from symptoms and risk factors:
Frequently asked questions
Calcium is normal but I have symptoms — why?
Total calcium depends on albumin: when protein is low, true hypocalcemia can "hide" behind a normal number. In such cases look at albumin-corrected or ionized calcium — it reflects the active fraction more accurately.
Should I take calcium for my bones?
Not everyone and not blindly. For bones the sum of factors matters more: enough vitamin D, protein, physical activity and calcium mainly from food. Excess calcium in supplements is not safe, so the decision is best based on labs and risk factors.
High calcium — is it cancer?
Not necessarily, but elevated calcium always needs to be sorted out. More than 90% of cases are primary hyperparathyroidism or cancer, so with persistent hypercalcemia parathyroid hormone is always checked and the cause is sought.
Related terms
Sources
- Serum Calcium — Clinical Methods (NCBI)
- Approach to Hypercalcemia — Endotext (NCBI)
- MedlinePlus (NIH) — Calcium Blood Test