Calcium: How to Read Your Blood Calcium Level

Calcium exists in blood in three forms:

• Ionized (free). About 50%. The biologically active fraction.
• Albumin-bound. About 40%. Inactive.
• Complexed. About 10%. Tied up with phosphate, citrate, bicarbonate.

The "calcium" on a CMP is the sum of all three. To estimate the active fraction when albumin is off:

Corrected calcium = measured calcium + 0.8 × (4.0 − albumin)

Worked example: measured 8.4, albumin 2.5. Corrected is 8.4 + 0.8 × 1.5 = 9.6. Normal. Without the correction, that patient looks hypocalcemic and isn't.

Ionized calcium (4.5 to 5.6 mg/dL, or 1.12 to 1.40 mmol/L) is the gold standard when precision matters: critically ill patients, complex acid-base disturbances, or any time the correction formula feels off.

• Total calcium (adults): 8.6–10.3 mg/dL (2.15–2.57 mmol/L). Slightly higher in children.
• Ionized calcium: 4.5–5.6 mg/dL (1.12–1.40 mmol/L).
• Albumin (for correction): 3.5–5.0 g/dL.
• Mild hypercalcemia: 10.4–11.9 mg/dL.
• Moderate hypercalcemia: 12.0–13.9 mg/dL.
• Severe hypercalcemia: above 14.0 mg/dL, emergency.
• Mild hypocalcemia: 7.6–8.5 mg/dL (corrected).
• Severe hypocalcemia: below 7.0 mg/dL, risk of tetany, seizures.

Two causes account for the great majority of hypercalcemia: primary hyperparathyroidism and malignancy. Sorting which is which is the work.

Primary hyperparathyroidism dominates the outpatient setting:

• Usually mild (10.5 to 11.5) and asymptomatic, found incidentally on a routine panel.
• PTH is inappropriately high or upper-normal despite the high calcium. (It should be suppressed.)
• Most cases are a single benign adenoma. Surgical cure rate is above 95% with an experienced parathyroid surgeon.
• Indications for parathyroidectomy: calcium above 11.5, age under 50, kidney stones, T-score below −2.5, eGFR below 60, hypercalciuria, or any symptoms.

Malignancy dominates the inpatient setting and tends to be more severe:

• Usually moderate to severe (above 12), and symptomatic.
• PTH is suppressed. PTHrP (the PTH-related peptide) may be elevated.
• Common cancers: breast, lung (especially squamous), multiple myeloma, renal cell, head and neck.
• Mechanisms: bone metastases, PTHrP secretion, or direct cytokine release.

Other causes worth knowing:

• Vitamin D toxicity (supplements above 10,000 IU/day for months) or granulomatous disease (sarcoidosis, TB).
• Thiazide diuretics. Usually modest, 10.4 to 10.8.
• Lithium. Chronic use mimics hyperparathyroidism.
• Milk-alkali syndrome. Heavy calcium-carbonate antacid use.
• Hyperthyroidism, Addison disease, immobilization (especially with Paget disease).
• Familial hypocalciuric hypercalcemia (FHH). Rare, but worth screening with a urine calcium/creatinine ratio before sending anyone to parathyroid surgery.

Classic symptom mnemonic, "stones, bones, groans, psychiatric overtones": kidney stones, bone pain, abdominal pain and constipation, polyuria, fatigue, depression, confusion. Severe hypercalcemia ends in lethargy and coma.

Step one: correct for albumin. Hypoalbuminemia is by far the most common cause of "low calcium" on a panel, and almost none of those patients are truly hypocalcemic.

Once albumin is accounted for, real hypocalcemia falls into a few buckets:

• Hypoparathyroidism. Most often post-surgical (after thyroidectomy or parathyroidectomy). Autoimmune is less common.
• Vitamin D deficiency. Especially in older adults, darker-skinned individuals, and northern latitudes. The leading cause of mild chronic hypocalcemia.
• Chronic kidney disease. Impaired vitamin D activation, phosphate retention, secondary hyperparathyroidism.
• Magnesium deficiency. Required for PTH secretion. Severe low magnesium causes functional hypoparathyroidism, and refractory hypocalcemia almost always means low magnesium until proven otherwise.
• Acute pancreatitis. Calcium soap formation in fat necrosis.
• Tumor lysis, rhabdomyolysis. Released phosphate binds calcium.
• Massive blood transfusion. Citrate binds ionized calcium.
• Pseudohypoparathyroidism, DiGeorge syndrome. Uncommon genetic causes.

Symptoms: perioral numbness, tingling in fingers and toes, muscle cramps, Chvostek sign (facial twitch with cheek tap), Trousseau sign (carpal spasm with a BP cuff inflated). Severe disease: tetany, seizures, prolonged QT, laryngospasm.

Calcium is one of the more stable lab values, so small drift over years rarely matters. What matters is steady upward drift, even within the "normal" range:

• Calcium creeping from 9.4 to 10.5 over five years. Order PTH, 25-OH vitamin D, and 24-hour urine calcium. This is how primary hyperparathyroidism is usually caught.
• New hypercalcemia at any age. PTH first, plus albumin or ionized calcium for confirmation.
• Falling calcium with rising creatinine. Secondary hyperparathyroidism in CKD. Check PTH, phosphate, and 25-OH vitamin D.
• Persistent low calcium after thyroid surgery. Post-surgical hypoparathyroidism. May need lifelong calcium and calcitriol.

For anyone on long-term calcium or vitamin D supplementation, periodic monitoring of calcium, 25-OH vitamin D, and (in older adults) kidney function is reasonable.

• Total calcium above 14 mg/dL. Emergency. Risk of dehydration, kidney injury, arrhythmia, and coma.
• Total calcium above 11.5. Urgent workup. Order PTH, 25-OH vitamin D, phosphate, albumin, creatinine, and 24-hour urine calcium.
• Mild persistent hypercalcemia (10.4–11.4). Order PTH on the same draw to distinguish primary hyperparathyroidism from PTH-independent causes.
• Corrected calcium below 7.5 mg/dL. Same-day evaluation. Risk of tetany and seizures.
• Symptomatic hypocalcemia at any level. Perioral numbness, cramps, paresthesias warrant evaluation.
• Refractory hypocalcemia. Check magnesium first. Replacement will not work without it.
• Calcium with kidney stones, osteoporosis, or fragility fracture. Screen for hyperparathyroidism with PTH.

• Albumin. Required for the corrected-calcium formula. Always interpret total calcium with albumin in view.
• Ionized calcium. The gold standard when accuracy matters or when albumin and acid-base are abnormal.
• PTH (intact). The single most useful test for working up hypercalcemia. High or upper-normal PTH with high calcium = primary hyperparathyroidism.
• 25-hydroxyvitamin D. Deficiency is the most common cause of mild hypocalcemia and contributes to secondary hyperparathyroidism.
• Magnesium. Required for PTH function; refractory hypocalcemia almost always involves magnesium.
• Phosphate. Moves opposite calcium in PTH-driven states; useful for distinguishing causes.
• 24-hour urine calcium. Distinguishes primary hyperparathyroidism (high) from familial hypocalciuric hypercalcemia (low).
• Creatinine and eGFR. Kidney function shapes calcium-phosphate balance.