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 ร (4 โ 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.
| 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) | 3ร 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). |
| MgSOโ | 2-4g IV over 20-60 min | IV | Correct Mg BEFORE Ca. Hypomagnesemia causes PTH resistance โ 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 ร 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). |
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, POโ 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 POโ = classic pattern. QTc prolongation โ arrhythmia risk.
Management:
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.
Patient: 34F with CKD stage 5 (not yet on dialysis). Generalized tonic-clonic seizure in ED. iCa 2.8 mg/dL, Mg 1.2, POโ 8.4, PTH 380 (elevated). Cr 7.8. QTc 560 ms.
Key findings: Severe hypocalcemia from CKD: elevated POโ (can't excrete), elevated PTH (secondary hyperparathyroidism but can't make active vitamin D), and hypomagnesemia (impairs PTH action). Seizure = emergent.
Management:
Teaching point: In CKD hypocalcemia, lowering phosphate is as important as giving calcium. High POโ ร Ca product > 55 โ metastatic calcification (soft tissue calcium deposits). Always correct Mg concurrently, it's futile to give calcium without adequate Mg.
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), POโ 2.0 (low), PTH 185 (elevated), 25-OH vitamin D 6 ng/mL (severely deficient). ALP 240 (elevated).
Key findings: Vitamin D deficiency โ low Ca AND low POโ (both malabsorbed). PTH appropriately elevated (secondary hyperparathyroidism). Elevated ALP = increased bone turnover (osteomalacia). This is the classic pattern: low Ca, low POโ, high PTH, high ALP.
Management:
Teaching point: The phosphate level is the key differentiator: low POโ = vitamin D deficiency (both Ca and POโ malabsorbed, PTH causes phosphaturia). High POโ = hypoparathyroidism or CKD (PTH normally wastes phosphate, without PTH, POโ rises).
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, POโ 5.2 (high), PTH < 5 (low). ECG: QTc 510 ms.