Hypocalcemia
Ionized calcium < 4.4 mg/dL or corrected total Ca < 8.5. Emergent when symptomatic (tetany, seizures, QTc prolongation). Most common causes: hypoparathyroidism (post-surgical), vitamin D deficiency, CKD.
🔍 Overview
Overview
Hypocalcemia = ionized Ca < 4.4 mg/dL or corrected total Ca < 8.5 mg/dL. Correct total calcium for albumin: corrected Ca = total Ca + 0.8 × (4 − albumin). Always check ionized calcium in critically ill patients (more accurate with hypoalbuminemia, acid-base disturbances). Most common causes: (1) Post-surgical hypoparathyroidism (after thyroidectomy/parathyroidectomy -most common inpatient cause), (2) Vitamin D deficiency (most common overall), (3) CKD (decreased 1,25-OH vitamin D production), (4) Hypomagnesemia (impairs PTH secretion AND causes PTH resistance -MUST correct Mg first), (5) Acute pancreatitis (calcium saponification), (6) Massive transfusion (citrate chelates calcium). Symptoms correlate with rate of decline more than absolute level.
🧪 Workup
Workup
- Ionized calcium -most accurate, especially in ICU (not affected by albumin or pH). < 4.4 mg/dL = hypocalcemia.
- Corrected total calcium -if ionized not available. Corrected = total + 0.8 × (4 − albumin).
- Albumin -for correction. Low albumin → falsely low total Ca (but ionized is normal).
- PTH (intact) -the diagnostic branch point. Low PTH = hypoparathyroidism (post-surgical, autoimmune, infiltrative). High PTH = secondary hyperparathyroidism (vitamin D deficiency, CKD, PTH resistance).
- Magnesium -MUST check. Mg < 1.5 → impairs PTH secretion AND causes end-organ PTH resistance. Hypocalcemia will NOT correct until Mg is repleted.
- 25-OH vitamin D -< 20 ng/mL = deficiency. Most common cause of hypocalcemia worldwide.
- 1,25-dihydroxy vitamin D -low in CKD (can't hydroxylate 25-OH to active form). Also low in hypoparathyroidism (PTH stimulates 1-alpha-hydroxylase).
- Phosphate -high PO₄ + low Ca = hypoparathyroidism or CKD. Low PO₄ + low Ca = vitamin D deficiency.
- ECG -prolonged QTc (risk of torsades de pointes). Also: ST changes mimicking ischemia.
- BMP, Cr -CKD assessment
🚨 Management
Management
- EMERGENT (symptomatic or iCa < 3.2):
- Calcium gluconate 10% 1-2 g (10-20 mL) IV over 10-20 min -first-line, and safe through a peripheral IV. Repeat until symptoms resolve.
- Calcium chloride 10% 1 g IV over 5-10 min through a central line only -it carries 3× the elemental calcium (272 mg/g vs 93), but extravasation causes tissue necrosis.
- Follow the bolus with an infusion -5-10 g calcium gluconate in 500 mL D5W over 12-24h. A bolus alone wears off within hours and the tetany returns.
- Continuous ECG monitoring -the prolonged QT is what precedes the arrhythmia, so it is the parameter that tells you the correction is working.
- Do NOT run calcium in the same line as bicarbonate or phosphate -they precipitate. Flush between, or use a separate line.
- Correct MAGNESIUM FIRST: Hypocalcemia will NOT respond to calcium supplementation if Mg is depleted. MgSO₄ 2-4g IV over 20-60 min.
- Chronic management:
- Vitamin D deficiency: Ergocalciferol 50,000 IU PO weekly × 8-12 wk → maintenance 1000-2000 IU daily
- Hypoparathyroidism: Calcitriol 0.25-2 mcg PO BID + calcium carbonate 1-3g TID with meals. Target: low-normal Ca (8-8.5) to avoid hypercalciuria.
- CKD: Calcitriol (active vitamin D) + phosphate binders. Correct 25-OH vitamin D if deficient. Manage per CKD-MBD guidelines.
Post-thyroidectomy "hungry bone syndrome": After parathyroidectomy for hyperparathyroidism, bones rapidly take up calcium → severe, prolonged hypocalcemia. May need IV calcium drip × days + high-dose oral calcium + calcitriol. Check Ca q6h post-op. Witteveen et al., 2013
💊 Medications
Medications
| Drug | Dose | Route | Notes |
|---|---|---|---|
| Calcium gluconate 10% | 1-2g (10-20 mL) IV over 10-20 min | IV | First-line IV. Can use peripheral IV. 93 mg elemental Ca per gram. Repeat PRN. Follow with drip. |
| Calcium chloride 10% | 1g (10 mL) IV over 5-10 min | IV (central) | 3× more elemental Ca than gluconate (272 mg/g). Central line ONLY -tissue necrosis risk. For arrest/severe tetany. |
| Ca gluconate drip | 5-10g in 500 mL D5W over 12-24h | IV | Continuous infusion for sustained correction. Do NOT mix with bicarb (precipitates). |
| MgSO₄ | 2-4g IV over 20-60 min | IV | Correct Mg BEFORE Ca. Hypomagnesemia causes PTH resistance → Ca won't correct. |
| Calcitriol | 0.25-2 mcg PO BID | PO | Active vitamin D. For hypoparathyroidism + CKD. Fast onset (1-2 days). Monitor Ca closely (narrow window). |
| Ergocalciferol (D2) | 50,000 IU weekly × 8-12 wk | PO | Vitamin D deficiency repletion. Then maintenance 1000-2000 IU daily. |
| Calcium carbonate | 500-1500 mg elemental TID with meals | PO | Chronic replacement. Requires gastric acid for absorption (take with food, not with PPI). |
🏥 Rounds
Pimp Questions
❓ Why must you correct magnesium before calcium?
Hypomagnesemia causes hypocalcemia through TWO mechanisms: (1) Impaired PTH secretion -Mg is required for PTH release from the parathyroid gland. (2) End-organ PTH resistance -Mg is needed for PTH to activate its receptor in bone and kidney. The result: giving calcium without correcting Mg is futile -the Ca will continue to drop because PTH is dysfunctional. Always check and correct Mg first.
❓ What is the difference between calcium gluconate and calcium chloride?
Calcium gluconate: 93 mg elemental Ca per gram. Can go through peripheral IV. Lower tissue necrosis risk. First-line for most situations. Calcium chloride: 272 mg elemental Ca per gram (3× more). Requires central line -causes severe tissue necrosis if peripheral IV infiltrates. Reserved for cardiac arrest, severe tetany, or when rapid correction needed via central access.
❓ What is Trousseau sign and why is it more specific than Chvostek sign?
Trousseau sign: inflate BP cuff above systolic × 3 min → carpal spasm (metacarpophalangeal flexion + interphalangeal extension + thumb adduction = "obstetrician's hand"). Specificity ~94% for hypocalcemia. Chvostek sign: tap facial nerve → ipsilateral facial muscle twitch. Specificity only ~30% -present in ~10% of normocalcemic individuals. Trousseau is far more reliable clinically.
❓ What is "hungry bone syndrome"?
After parathyroidectomy for severe hyperparathyroidism, the chronically suppressed bones suddenly have normal PTH levels → rapid uptake of calcium, phosphate, and magnesium into demineralized bone → severe, prolonged hypocalcemia (can last days to weeks). Risk factors: high pre-op PTH, high pre-op ALP, vitamin D deficiency, high pre-op Ca. Treatment: aggressive IV calcium drip + high-dose oral calcium + calcitriol. May need 10+ grams of elemental calcium per day initially.
❓ How does the phosphate level help differentiate causes of hypocalcemia?
High PO₄ + low Ca: hypoparathyroidism (PTH normally causes renal phosphate wasting -without PTH, PO₄ rises) or CKD (can't excrete PO₄). Low PO₄ + low Ca: vitamin D deficiency (both Ca and PO₄ are low due to impaired gut absorption) -PTH is appropriately elevated (secondary hyperparathyroidism), which causes phosphaturia. Phosphate is a crucial clue that most people forget to check.
❓ Why should you NOT mix IV calcium with bicarbonate?
Calcium and bicarbonate form calcium carbonate precipitate (insoluble chalk) in the IV tubing. This can cause tubing occlusion and -more dangerously -if the precipitate enters the bloodstream, it can cause embolic tissue damage. Always flush lines between calcium and bicarb. Same applies to calcium + phosphate-containing solutions.
Clinical Examples
📋 Case 1, Post-Thyroidectomy Hypocalcemia
Patient: 48F, POD1 from total thyroidectomy for papillary thyroid carcinoma. Perioral tingling, hand cramping. HR 94, BP 130/78. Positive Trousseau and Chvostek signs. iCa 3.4 mg/dL, Mg 1.9, PO₄ 5.4, PTH < 5. QTc 520 ms.
Key findings: Post-surgical hypoparathyroidism, most common cause of acute hypocalcemia in hospital. Low PTH + low Ca + high PO₄ = classic pattern. QTc prolongation → arrhythmia risk.
Management:
- Calcium gluconate 2g IV over 20 min → start continuous drip (6g in 500 mL D5W at 50 mL/hr)
- Calcitriol 0.5 mcg PO BID (active vitamin D, bypasses PTH-dependent 1-alpha-hydroxylation)
- Oral calcium carbonate 1-2g TID with meals (long-term replacement)
- Check iCa q4-6h until stable on drip. Target iCa > 4.0 mg/dL
- Telemetry for QTc monitoring, torsades risk if QTc > 500 ms
Teaching point: Use calcitriol (not ergocalciferol) in hypoparathyroidism, without PTH, the kidney cannot 1-alpha-hydroxylate 25-OH vitamin D to its active form. Calcitriol is already active and works within hours.
📋 Case 2, Severe Symptomatic Hypocalcemia with Seizure
Patient: 34F with CKD stage 5 (not yet on dialysis). Generalized tonic-clonic seizure in ED. iCa 2.8 mg/dL, Mg 1.2, PO₄ 8.4, PTH 380 (elevated). Cr 7.8. QTc 560 ms.
Key findings: Severe hypocalcemia from CKD: elevated PO₄ (can't excrete), elevated PTH (secondary hyperparathyroidism but can't make active vitamin D), and hypomagnesemia (impairs PTH action). Seizure = emergent.
Management:
- Calcium gluconate 4g IV over 20 min (2 amps in code-like urgency for seizure)
- Magnesium sulfate 2g IV over 1h, MUST correct Mg first or Ca will not stay up
- Continuous calcium drip after bolus, iCa q2h until > 4.0
- Sevelamer 800 mg TID with meals (phosphate binder, high PO₄ complexes with Ca, worsening hypocalcemia)
- Calcitriol 0.25-0.5 mcg daily. Expedite dialysis initiation (nephrologist)
Teaching point: In CKD hypocalcemia, lowering phosphate is as important as giving calcium. High PO₄ × Ca product > 55 → metastatic calcification (soft tissue calcium deposits). Always correct Mg concurrently, it's futile to give calcium without adequate Mg.
📋 Case 3, Vitamin D Deficiency Hypocalcemia
Patient: 72F from nursing home, minimal sun exposure, poor nutrition. Progressive fatigue, muscle cramps, diffuse bone pain. Ca 7.8, albumin 3.8 (corrected Ca 8.0), PO₄ 2.0 (low), PTH 185 (elevated), 25-OH vitamin D 6 ng/mL (severely deficient). ALP 240 (elevated).
Key findings: Vitamin D deficiency → low Ca AND low PO₄ (both malabsorbed). PTH appropriately elevated (secondary hyperparathyroidism). Elevated ALP = increased bone turnover (osteomalacia). This is the classic pattern: low Ca, low PO₄, high PTH, high ALP.
Management:
- Ergocalciferol (D2) 50,000 IU PO weekly × 8-12 weeks, then 1000-2000 IU daily maintenance
- Oral calcium carbonate 1-1.5g daily (gut absorption impaired without vitamin D, will improve as D repletes)
- Recheck 25-OH vitamin D at 8-12 weeks, target > 30 ng/mL
- Monitor Ca, PO₄, PTH, PTH should normalize as vitamin D repletes
- DEXA scan once vitamin D replete (osteomalacia artificially lowers T-score)
Teaching point: The phosphate level is the key differentiator: low PO₄ = vitamin D deficiency (both Ca and PO₄ malabsorbed, PTH causes phosphaturia). High PO₄ = hypoparathyroidism or CKD (PTH normally wastes phosphate, without PTH, PO₄ rises).
Sample Presentation
Mrs. Liu is a 55-year-old woman, post-op day 1 from total thyroidectomy for papillary thyroid carcinoma. Nurse calls for perioral tingling and hand cramping. VS: HR 92, BP 128/78. Exam: positive Trousseau sign, positive Chvostek sign. Labs: iCa 3.6 mg/dL (low), Mg 1.8, PO₄ 5.2 (high), PTH < 5 (low). ECG: QTc 510 ms.
Key Points: Post-surgical hypoparathyroidism (most common cause of acute hypocalcemia in hospital). Low PTH + low Ca + high PO₄ = classic. QTc prolonged → emergent. Give calcium gluconate 2g IV over 20 min → start drip. Start calcitriol 0.5 mcg BID + calcium carbonate 1g TID. Check iCa q6h. This may be transient (parathyroids stunned) or permanent.
- Ionized calcium q4-6h during IV replacement (q2h if critically symptomatic)
- ECG -QTc monitoring. QTc should shorten as Ca normalizes. Torsades risk if QTc > 500 ms.
- Mg level -recheck after repletion. Must remain > 1.5 for Ca correction to work.
- Post-thyroidectomy: ionized Ca q6h × 24-48h. Albumin-corrected Ca unreliable post-surgery (fluid shifts). ATA Thyroidectomy Guidelines, 2020
- Phosphate -trending helps differentiate causes. Falling PO₄ + rising Ca on treatment = responding to vitamin D/PTH replacement.
- Chvostek and Trousseau signs -clinical bedside assessment. Chvostek: tap facial nerve → ipsilateral facial twitch. Trousseau: BP cuff inflated above systolic × 3 min → carpal spasm (more specific).
- 24h urine calcium -for chronic hypoparathyroidism. Target: avoid hypercalciuria (> 300 mg/24h) → nephrolithiasis risk.
- Cr -calcium + vitamin D therapy can worsen renal function. Monitor in CKD patients.
📋 Summary
Summary
Confirm It Is Real
Check ionized calcium, or correct the total for albumin: add 0.8 mg/dL for every 1 g/dL the albumin sits below 4. Hypoalbuminemia is the commonest cause of a low total calcium with a completely normal ionized level, and treating that number is treating an artifact. Alkalosis also lowers ionized calcium at any given total, which is why hyperventilation causes tetany.
Recognize the Emergency
Tetany, carpopedal spasm, laryngospasm, seizures and a prolonged QTc mean treat now. Chvostek and Trousseau signs support it (Trousseau is the more specific). Laryngospasm and arrhythmia are what kill, so symptomatic hypocalcemia is treated before the workup is complete.
Give IV Calcium the Right Way
Calcium gluconate 1 to 2 g IV over 10 to 20 min is first line and can go through a peripheral line. Calcium chloride has 3 times the elemental calcium but is central line only, because extravasation causes tissue necrosis. Follow the bolus with an infusion, 5 to 10 g of gluconate in 500 mL D5W over 12 to 24 h, since a bolus alone wears off within hours. Never run calcium in the same line as bicarbonate or phosphate: it precipitates.
Fix the Magnesium First
Hypomagnesemia causes both PTH resistance and impaired PTH secretion, so the calcium will not correct until the magnesium does. Give magnesium sulfate 2 to 4 g IV. This is the single commonest reason refractory hypocalcemia stays refractory, and it is often missed in alcohol use disorder, diuretic use and proton pump inhibitor therapy.
Work Out the Mechanism From the PTH
Low or inappropriately normal PTH → hypoparathyroidism, most often post-surgical, or magnesium-related. High PTH → the gland is responding appropriately, so look for vitamin D deficiency, chronic kidney disease, or pseudohypoparathyroidism. Check magnesium, phosphate, creatinine and 25-hydroxyvitamin D at the same time.
Match the Vitamin D to the Problem
Calcitriol 0.25 to 2 mcg PO BID for hypoparathyroidism and CKD, because both lack renal 1-alpha-hydroxylase activity and cannot activate plain vitamin D. Ergocalciferol 50,000 IU weekly for 8 to 12 weeks for simple vitamin D deficiency, then 1,000 to 2,000 IU daily maintenance.
Anticipate the High-Risk Situations
Post-thyroidectomy and post-parathyroidectomy hungry bone syndrome: the skeleton takes up calcium rapidly, producing severe, prolonged hypocalcemia that may need a calcium infusion for days plus high-dose oral calcium and calcitriol. Also expect it after massive transfusion (citrate chelation), in pancreatitis, in tumor lysis syndrome, and with bisphosphonates or denosumab given to a vitamin D deficient patient.
Do Not Chase Calcium in Tumor Lysis
Treat the hypocalcemia of tumor lysis syndrome only if the patient is symptomatic. Giving calcium while the phosphate is high drives calcium-phosphate precipitation in the kidney and worsens the acute kidney injury. Lower the phosphate first.
📄 One Pager
Text version
One-Pager
Hypocalcemia
Correct Mg first. Always.
Emergent Treatment
Ca gluconate 1-2g IV over 10-20 min (peripheral OK). Follow with drip 5-10g/24h. Ca chloride via central only (3× more elemental Ca). MgSO₄ 2-4g IV if Mg < 1.5.
Diagnostic Branch
PTH low → hypoparathyroidism (post-surgical, autoimmune). PTH high → secondary (vitamin D deficiency, CKD). PO₄ helps: high PO₄ = hypopara/CKD. Low PO₄ = vitamin D deficiency.
Bedside Tests
Chvostek (tap facial nerve → twitch, 30% specific). Trousseau (BP cuff → carpal spasm, 94% specific). ECG: QTc prolongation → torsades risk.
Related Topics
Adrenal CrisisCushing's SyndromeDiabetes InsipidusHypercalcemiaHyperosmolar Hyperglycemic State (HHS)Hyperthyroidism / Graves' Disease