Calcium

What is Calcium?

Calcium is the most abundant mineral in the human body — around 99% is stored in bones and teeth, with only 1% circulating in the blood and soft tissues. It is essential for muscle contraction (including the heart), nerve conduction, blood clotting, and the building and maintenance of bone mass. Blood calcium is reported in mmol/L (internationally also mg/dL; 1 mmol/L ≈ 4 mg/dL) and is tightly regulated by a triangle of hormones: parathyroid hormone (PTH) from the parathyroid glands, active vitamin D (calcitriol), and calcitonin. Roughly half the calcium in the blood circulates freely (ionised, the physiologically active fraction), and the other half is bound — mainly to albumin. Total serum calcium measures both fractions together. This makes albumin a determining factor: with low albumin, total calcium falls even when free calcium is normal. Corrected calcium accounts for this; ionised calcium measures the active fraction directly and is the most reliable measure when in doubt.

Why is Calcium relevant?

Blood calcium is regulated with extreme precision — the body sacrifices bone mass to maintain blood calcium levels. That means a genuinely abnormal calcium is almost never simply due to too little or too much calcium in the diet, but always signals a disruption in the regulatory system itself. High calcium (hypercalcaemia) is most often caused by primary hyperparathyroidism or in people with cancer; low calcium (hypocalcaemia) by vitamin D deficiency, hypoparathyroidism, or kidney insufficiency. In everyday practice, the most relevant scenario is vitamin D deficiency — which impairs calcium absorption in the gut and disturbs bone remodelling, and over time can contribute to osteoporosis, muscle cramps, and elevated fracture risk. Measuring total calcium as part of a routine panel is therefore a useful screen of overall calcium homeostasis, with the caveat that abnormal values are always read alongside albumin and vitamin D.

Calcium high or low — what it means

Read total calcium alongside albumin and vitamin D. If albumin is low — from illness, undernutrition, or chronic inflammation — total calcium may also appear low even though active free calcium is normal. Corrected calcium adjusts for this using the formula: corrected calcium = measured calcium + 0.02 × (40 − albumin in g/L). When sustained doubt exists or symptoms are present (muscle cramps, tingling, sleeplessness, confusion), ionised calcium is the most direct measure. Reference ranges for total calcium are typically 2.15–2.55 mmol/L at most laboratories, but always check your own lab's bound. Values outside this range, confirmed on two measurements, call for further differentiation: PTH, vitamin D, phosphate, and kidney markers help identify the cause. A single abnormal calcium without symptoms almost always calls for a repeat before drawing conclusions.

Educational information only — not medical advice. Consult a healthcare professional for clinical decisions.