Around 99 percent of your calcium sits in your bones and teeth. Only 1 percent circulates in your blood. That single percent is guarded closely, because it drives your heartbeat, your muscle contraction and your blood clotting.
This has a consequence that surprises many people: your blood calcium says little about the strength of your bones. Your body keeps that blood number steady, if need be by pulling calcium out of your skeleton. A normal result therefore does not rule out bone loss.
What a calcium value can pick up is a disturbance in the regulation itself. That is where it gets interesting.
What is a normal calcium value?
Most laboratories use roughly 2.15 to 2.55 mmol/l for total serum calcium. Above that, doctors speak of hypercalcaemia; below it, hypocalcaemia. As with other values, the reference differs slightly per lab, so always compare against the range on your own result.
More important than the exact cut-off is how that number comes about. Roughly half your calcium travels bound to the protein albumin; the other half is free and active. A lab measures the total by default, so both parts together.
Why does albumin matter for your calcium value?
Because albumin can distort your calcium result without anything being wrong. If your albumin falls, for instance with liver disease or prolonged illness, your total calcium falls with it while the active, free calcium stays perfectly normal. The result looks low, but your body notices nothing.
That is why laboratories work with a corrected calcium value, adjusting the total for your albumin. The principle is not new: it was described back in 1973 (Payne 1973, PMID 4758544) and is still in use.
Imagine two people, both with a total calcium of 2.10 mmol/l. One has a normal albumin, the other a reduced albumin of 30 grams per litre. In the first, the value really is on the low side. In the second, the corrected value is probably simply normal. Same number, opposite meaning.
To my mind, this is exactly why you should never read a calcium result in isolation. If albumin is not on your result, ask for it.
What does too much calcium in your blood mean?
A raised calcium is rarely a dietary matter. It usually points to a disturbance in the hormonal regulation, and the best known cause is an overactive parathyroid gland. Those glands make the hormone that pulls calcium out of your bones and keeps it in your blood.
Symptoms are often absent. When they do appear, they are vague: passing a lot of urine, thirst, constipation, fatigue and sometimes kidney stones. A raised calcium is frequently found by chance on a routine test, which is precisely why it is taken seriously (Minisola 2015, PMID 26037642).
| Possible cause | What happens |
|---|---|
| Overactive parathyroid | More hormone pulls calcium out of your bones |
| Prolonged immobility | Bone breakdown releases calcium into the blood |
| Certain medication | Including lithium and some diuretics |
| Very high supplement intake | Rare, usually at extreme doses with vitamin D |
| Underlying illness | Sometimes with certain conditions, always investigated further |
A raised value is a reason for a conversation with your GP, not for panic. The next step is usually a repeat test plus a parathyroid hormone measurement.
What does a calcium shortage mean?
A genuinely low blood calcium is less common than people assume, because your body corrects it quickly. What people call a "calcium shortage" is usually about too little calcium in the diet, and that is exactly what your blood does not show. It comes out of your bone store instead.
The Dutch Voedingscentrum holds that adults need roughly 950 to 1,000 milligrams of calcium a day. Dairy, leafy greens, nuts and pulses supply most of it. Leaving out dairy entirely removes a substantial source.
Vitamin D plays doorman here: without enough vitamin D you absorb calcium less well. The two belong together. Read on about vitamin D deficiency if you want to know more.
When do you have your calcium measured?
Measuring is mainly useful with symptoms or after a previously abnormal result. Think of kidney stones, bone complaints, heavy urination with thirst, or an incidentally found high value you want to repeat. Without symptoms and on a varied diet, a stand-alone calcium measurement usually adds little.
I always look at albumin first before I make anything of a calcium value. Without that number, the rest is a guess.
At Vitalcheck, calcium is read together with albumin and phosphate, because those three explain each other. You can have calcium and phosphate measured in an extended health checkup, without a referral.
For how calcium relates to the other minerals, read the pillar on minerals and trace elements. For the closely related salt balance, your potassium value is the logical follow-up. Discuss any abnormal result with your GP.
References
- Minisola S, et al. The diagnosis and management of hypercalcaemia. BMJ. 2015. PMID 26037642.
- Payne RB, et al. Interpretation of serum calcium in patients with abnormal serum proteins. Br Med J. 1973. PMID 4758544.
- Costello RB, et al. Perspective: The Case for an Evidence-Based Reference Interval for Serum Magnesium. Adv Nutr. 2016. PMID 28140318.
- Voedingscentrum. Calcium.
- Gezondheidsraad. Dietary reference values for calcium and vitamin D.
Every blood test result includes a professional assessment from a BIG-registered doctor. For treatment decisions, discuss your results with your GP.
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