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MineralsBone Health

Calcium

Mineral essential for bone formation, muscle function, and nerve signaling.

What is Calcium?

Calcium is the most abundant mineral in your body. Almost all of it is stored in your bones and teeth; only a small fraction circulates in your blood. It is essential for muscle contraction (including the heart), nerve signalling, blood clotting, and maintaining bone mass. The parathyroid glands and vitamin D tightly regulate the blood level. About half of blood calcium is bound to albumin. Total serum calcium measures both bound and free fractions together. If albumin is low, total calcium appears low too — even if the physiologically active free calcium is normal. Corrected calcium adjusts for that.

Why is Calcium relevant?

Your body keeps blood calcium within an extremely narrow range. When it falls outside that range, it almost always signals a disruption in the regulatory system — not too much or too little calcium in your diet. Vitamin D deficiency is the most common driver: it impairs calcium absorption in the gut and disrupts bone remodelling, which over time can increase the risk of muscle cramps and fractures. A persistently high calcium always warrants medical attention. So does a persistently low one. In both cases, additional markers — PTH and vitamin D — help pinpoint where the disruption sits.

Calcium high or low — what it means

Always read total calcium alongside albumin and vitamin D. An abnormal calcium without that context has limited meaning. If albumin is low, read corrected calcium rather than total — that gives a fairer picture of the physiologically active fraction. When symptoms like muscle cramps, tingling, or confusion are present, ionised calcium is the most direct measure. A single abnormal value without symptoms calls for a repeat before drawing conclusions. A value that is consistently outside the reference range, without a clear cause, is a reason to check PTH, vitamin D, and kidney function.

Calcium reference ranges

Normal (adults)Total serum calcium. Read alongside albumin.2.10-2.55 mmol/L
Ionised (free) calcium - venousThe directly measured, biologically active fraction; more reliable with symptoms or abnormal albumin.1.15-1.33 mmol/L

Cut-offs vary by lab and method (some Dutch labs use e.g. 2.20-2.65 mmol/L for adults). Total calcium is only meaningful read together with albumin; when albumin is low, corrected or ionised calcium gives a fairer picture.

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

Read about our scientific approach →Read the guide: Supplements →

Frequently asked questions

What is a normal blood calcium level?

For adults, total serum calcium usually sits between 2.10 and 2.55 mmol/L (some Dutch labs use 2.20-2.65 mmol/L). Ionised, free calcium is 1.15-1.33 mmol/L in venous blood. Cut-offs differ by lab and method; always read total calcium together with your albumin.

What does a high calcium mean?

A value above roughly 2.55 mmol/L (hypercalcaemia) almost never reflects too much dietary calcium, but a disruption in regulation. The best-known cause is raised parathyroid hormone (PTH), sometimes from a benign growth in a parathyroid gland. Other causes include cancer spread to the bones, an overactive thyroid, sarcoidosis, prolonged immobility, or too many vitamin D tablets. A persistently high calcium always warrants medical attention.

What does a low calcium mean?

A value below roughly 2.10 mmol/L (hypocalcaemia) is most often caused by low protein (albumin) in the blood: total calcium then looks low while the free, active calcium is normal, so there is no real deficiency. That is why we calculate a corrected calcium when albumin is low (often < 35 g/L). Other causes include vitamin D deficiency, low PTH production, or undernutrition.

When is an abnormal calcium concerning?

A single abnormal value without symptoms usually calls for a repeat measurement rather than alarm. It becomes more concerning if calcium stays abnormal on two measurements without a clear cause, or if there are symptoms such as muscle cramps, tingling, extreme thirst, or confusion. Then the logical next step is follow-up with PTH, vitamin D, and kidney function.

Related biomarkers

Albumin
Albumin is the most common protein in your blood that retains fluid and transports substances through your body.
Vitamin D
Vitamin D is a vitamin your skin mainly makes under sunlight that supports your bones, muscles, and immune defence.
Magnesium
Magnesium is a mineral involved in energy production, muscle function, and nerve signalling.
Potassium
Potassium is an electrolyte that is essential for your heart rhythm, muscle function and nerve signaling.

Calcium is one of the biomarkers in the Optimize blood test. Book a blood draw at any of 238+ partner labs in the Netherlands, or upload your existing results in the app.

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