Calcium
Calcium is the most abundant mineral in your body — about 99% resides in bones and teeth, while the remaining 1% circulates in blood performing critical functions for muscle contraction, nerve transmission, heart rhythm, and blood clotting. Blood levels are tightly regulated: your body will pull this mineral from bones to maintain blood levels, so normal results don’t guarantee healthy bones. Abnormal levels signal serious conditions including parathyroid disorders, vitamin D deficiency, kidney disease, or malignancy.
Calcium is the most abundant mineral in your body — about 99% resides in bones and teeth, while the remaining 1% circulates in blood where it performs critical functions. This small circulating fraction is tightly regulated because it’s essential for muscle contraction, nerve transmission, heart rhythm, blood clotting, and cellular signaling.
Why does testing matter? Because your body prioritizes blood levels so strongly that it will pull this mineral from bones to maintain them — meaning blood levels can appear normal even while bones are depleting. Abnormal levels signal serious conditions: parathyroid disorders, vitamin D deficiency, kidney disease, or certain cancers. Both high and low values cause significant symptoms and health consequences.
This blood test is essential for evaluating bone health, investigating symptoms like muscle cramps or fatigue, monitoring parathyroid function, and assessing people with kidney disease or certain chronic conditions. It’s one of the most commonly ordered tests and provides crucial metabolic information.
Key Benefits of Testing
A blood test reveals whether your levels are within the narrow range needed for proper body function. Both elevated and decreased values cause symptoms and indicate underlying conditions requiring treatment. Testing detects parathyroid disorders — the most common cause of high levels — often before symptoms become severe.
This assessment also helps evaluate bone health, though normal blood levels don’t guarantee healthy bones (the body maintains blood levels at the expense of bone). Combined with vitamin D and parathyroid hormone (PTH), it provides comprehensive insight into mineral metabolism. For people with kidney disease, cancer, or on certain medications, monitoring prevents dangerous imbalances.
What Does This Test Measure?
The standard blood test measures total calcium — all the mineral circulating in your blood. This exists in three forms:
Protein-bound (about 40%): Attached primarily to albumin. This form is inactive — it’s essentially being transported.
Ionized/free (about 50%): The biologically active form that affects muscles, nerves, and heart. This is what matters physiologically.
Complexed (about 10%): Bound to small molecules like phosphate and citrate.
Total vs. Corrected Testing
Most labs measure total levels, which works well when albumin is normal. However, since 40% is bound to albumin, abnormal albumin affects results. When albumin is low, total levels appear falsely decreased; when albumin is high, they appear falsely elevated — even though the active ionized fraction hasn’t changed.
For accurate interpretation when albumin is abnormal, a corrected (adjusted) value compensates for albumin changes. If your albumin is significantly abnormal, ask your provider about corrected results.
Regulation of Blood Levels
Your body tightly regulates circulating levels through three hormones:
Parathyroid hormone (PTH): Released when levels drop. Increases bone release, kidney retention, and vitamin D activation.
Vitamin D: Increases intestinal absorption. Works with PTH to maintain levels.
Calcitonin: Released when levels are high. Has modest lowering effects.
Why This Test Matters
Detects Parathyroid Disorders
The parathyroid glands regulate blood levels by releasing PTH. Overactive parathyroid (hyperparathyroidism) is the most common cause of elevated levels. Testing detects this condition, which causes bone loss, kidney stones, fatigue, and other problems if untreated.
Evaluates Bone Metabolism
While blood levels don’t directly show bone density, abnormalities indicate problems with mineral metabolism affecting bones. High levels from hyperparathyroidism cause bone loss. Low vitamin D impairs absorption and weakens bones. This test is part of comprehensive bone health evaluation.
Identifies Serious Conditions
Significantly elevated levels (hypercalcemia) can indicate malignancy — certain cancers release substances that raise blood levels. Severely low levels (hypocalcemia) can cause dangerous muscle spasms and heart rhythm problems. Both require prompt evaluation.
Monitors High-Risk Individuals
People with kidney disease, those on certain medications (thiazide diuretics, lithium), or with known parathyroid issues need regular monitoring to prevent dangerous imbalances.
What Can Affect Your Levels?
Causes of High Levels (Hypercalcemia)
Primary hyperparathyroidism: The most common cause. A parathyroid adenoma (benign growth) overproduces PTH, raising blood levels.
Malignancy: A common cause in people with cancer. Tumors can raise levels through bone spread or by producing PTH-related substances.
Excess vitamin D: Oversupplementation or certain inflammatory conditions increase absorption.
Thiazide diuretics: Reduce urinary excretion, raising blood levels.
Immobilization: Prolonged bed rest releases mineral from bones.
Dehydration: Concentrates the blood, artificially elevating total levels.
Causes of Low Levels (Hypocalcemia)
Vitamin D deficiency: Impairs intestinal absorption. Very common cause of low levels.
Hypoparathyroidism: Underactive parathyroid glands (often after thyroid surgery) reduce PTH, lowering blood levels.
Kidney disease: Impairs vitamin D activation and causes phosphate retention, both lowering levels.
Low magnesium: Impairs PTH function and causes resistance to PTH. Must correct magnesium to correct this mineral.
Pancreatitis: Acute pancreatitis can cause significant drops.
Certain medications: Bisphosphonates, some chemotherapy agents, and others can lower levels.
Testing Considerations
Fasting not typically required. Prolonged tourniquet use during blood draw can falsely elevate results. Always interpret with albumin — if albumin is abnormal, use corrected values. Recent vitamin D supplementation affects results.
When Should You Get Tested?
Symptoms of Imbalance
High levels may cause: Fatigue, weakness, confusion, excessive thirst and urination, constipation, nausea, bone pain, kidney stones.
Low levels may cause: Muscle cramps or spasms, numbness/tingling (especially around mouth and fingers), fatigue, brittle nails, confusion, irregular heartbeat.
Bone Health Concerns
If you have osteoporosis, osteopenia, or risk factors for bone loss, this test helps evaluate mineral metabolism as part of comprehensive assessment.
Parathyroid Evaluation
If parathyroid disorder is suspected — elevated levels on previous testing, kidney stones, unexplained bone loss — testing with PTH provides diagnosis.
Kidney Disease
Chronic kidney disease disrupts mineral metabolism. Regular monitoring helps guide treatment and prevent complications.
Certain Medications
If taking thiazide diuretics, lithium, vitamin D supplements, or other medications affecting mineral balance, periodic monitoring is appropriate.
Malignancy Workup
Elevated levels without clear cause warrant evaluation for underlying malignancy, as high calcium can be the first sign of cancer.
Understanding Your Results
Your lab provides reference ranges. Always check if the result is total or corrected, and note your albumin level:
Low (hypocalcemia): Below reference range. If confirmed on corrected value, indicates true deficiency. Evaluate for vitamin D deficiency, parathyroid problems, kidney disease, or magnesium deficiency.
Normal: Within reference range. Indicates adequate circulating levels — though doesn’t guarantee bone health.
High (hypercalcemia): Above reference range. Requires evaluation for cause: check PTH to distinguish parathyroid causes from others. Mild elevation may be monitored; significant elevation requires prompt workup.
Interpreting with PTH
The relationship between this mineral and PTH is crucial for diagnosis:
High calcium + High PTH: Primary hyperparathyroidism (parathyroid problem)
High calcium + Low PTH: Non-parathyroid cause (malignancy, vitamin D excess, etc.)
Low calcium + High PTH: Secondary hyperparathyroidism (parathyroid responding appropriately to low levels — look for vitamin D deficiency, kidney disease)
Low calcium + Low PTH: Hypoparathyroidism (parathyroid failure)
Don’t Forget Albumin
If albumin is low and total level appears low, check the corrected value — the true ionized fraction may be normal. This is especially important in older adults and those with chronic conditions.
What to Do About Abnormal Results
For Low Levels
Identify the cause: Check vitamin D, PTH, magnesium, and kidney function. Treatment depends on cause.
Vitamin D deficiency: Supplement vitamin D. Levels will improve as absorption normalizes.
Low magnesium: Correct magnesium first — levels won’t normalize until magnesium is adequate.
Hypoparathyroidism: Requires vitamin D and supplementation, managed by your healthcare provider.
Dietary sources: Dairy products, fortified foods, leafy greens, sardines with bones. Diet alone rarely corrects significant deficiency but supports treatment.
For High Levels
Determine the cause: PTH level distinguishes parathyroid from non-parathyroid causes.
Primary hyperparathyroidism: Often monitored if mild; surgery may be recommended for significant elevation or complications (kidney stones, bone loss).
Malignancy-related: Treat underlying cancer. Acute management may include IV fluids and medications.
Medication-related: Adjust offending medication if possible.
Hydration: Ensure adequate fluid intake; dehydration worsens elevated levels.
Related Health Conditions
Parathyroid Disorders
Hyperparathyroidism: Overactive parathyroid causing elevated levels, bone loss, and kidney stones.
Hypoparathyroidism: Underactive parathyroid causing low levels and muscle symptoms.
Bone Health
Osteoporosis: Mineral metabolism abnormalities contribute to bone loss. Testing is part of comprehensive evaluation.
Kidney Disease
Chronic Kidney Disease: Disrupts mineral metabolism, often causing low levels and secondary hyperparathyroidism.
Vitamin D Deficiency
Low Vitamin D: Common cause of low levels. Often corrected with supplementation.
Why Regular Testing Matters
For those with known parathyroid disorders, kidney disease, or on affecting medications, regular monitoring prevents dangerous imbalances. Levels can change gradually, and catching trends early allows intervention before complications develop.
Even in healthy individuals, this test as part of routine panels provides valuable metabolic information and can detect problems — like parathyroid adenomas — years before symptoms become significant.
Related Biomarkers Often Tested Together
Calcium Adjusted (Corrected) — Accounts for albumin abnormalities. Essential when albumin is low or high.
Parathyroid Hormone (PTH) — Essential for interpreting abnormal results. Distinguishes parathyroid from non-parathyroid causes.
Vitamin D — Required for absorption. Low vitamin D is a common cause of low levels.
Albumin — Needed to calculate corrected values when albumin is abnormal.
Magnesium — Low magnesium prevents normalization. Often tested together.
Phosphorus — Inversely related; important for complete mineral metabolism picture.
Creatinine — Kidney function affects mineral metabolism. Important context.
Note: Information provided in this article is for educational purposes and doesn’t replace personalized medical advice.
Frequently Asked Questions
Total measures all circulating mineral, including the portion bound to albumin. Since about 40% binds to albumin, abnormal albumin affects total results. Corrected (adjusted) mathematically compensates for abnormal albumin, estimating what total would be if albumin were normal. When albumin is normal, both values are essentially the same.
When albumin is significantly abnormal — especially low. Low albumin makes total levels appear falsely low. If your albumin is normal, total and corrected results will be nearly identical and either can be used.
Yes — this is common. Your body prioritizes blood levels, pulling mineral from bones to maintain them. You can have normal blood levels while losing bone density. Blood testing alone doesn’t diagnose osteoporosis; bone density testing is needed.
The two most common causes are hyperparathyroidism (parathyroid overactivity) and malignancy. Other causes include excess vitamin D, certain medications, and dehydration. PTH level helps distinguish the cause.
Vitamin D deficiency is the most common cause. Others include hypoparathyroidism, kidney disease, and low magnesium. Treatment depends on the underlying cause.
Fasting is not typically required. However, some labs prefer fasting for consistency. Recent meals don’t significantly affect results.
Ionized (free) testing directly measures the biologically active form — the portion not bound to proteins. It’s more accurate than total or corrected in certain situations (critically ill patients, abnormal blood proteins) but requires special handling and is not routine.
For routine screening: as part of comprehensive metabolic panels when indicated. For known parathyroid or kidney disease: as directed by your provider, often every few months. For monitoring treatment: based on clinical situation.
References
Key Sources:
- Bilezikian JP, et al. Primary Hyperparathyroidism. Nat Rev Dis Primers. 2016;2:16033.
- Cooper MS, Gittoes NJ. Diagnosis and management of hypocalcaemia. BMJ. 2008;336(7656):1298-1302.
- Minisola S, et al. The diagnosis and management of hypercalcaemia. BMJ. 2015;350:h2723.
