Hypocalcemia
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Hypocalcemia
Definition
Hypocalcemia is defined as a serum total calcium < 8.5 mg/dL (< 2.12 mmol/L) or an ionized calcium < 2.0 mEq/L (< 1.0 mmol/L). Because roughly 50% of calcium is bound to albumin, a "corrected" calcium must be calculated when hypoalbuminemia is present:
Corrected Ca = Measured Ca + 0.8 × (4.0 - serum albumin)
The ionized (free) calcium is the physiologically active fraction and is not affected by albumin changes. Symptoms correlate more with the ionized calcium level and the rate of fall than with the total calcium alone. - Rosen's Emergency Medicine, p. 2524
Physiology Recap
Calcium regulation involves a feedback loop:
- PTH is released when calcium falls - it stimulates bone resorption, renal calcium reabsorption, and 1,25(OH)₂D (calcitriol) synthesis.
- Vitamin D (calcitriol) promotes intestinal calcium absorption.
- Calcitonin (from thyroid C cells) opposes calcium-raising effects.
Disruption at any point produces hypocalcemia. - Rosen's Emergency Medicine
Causes / Etiology
1. Hypoparathyroidism (most common cause in clinical practice)
- Post-surgical - occurs in 1-2% of total thyroidectomies; also after parathyroidectomy or head/neck surgery for cancer
- Autoimmune - isolated or as part of autoimmune polyglandular syndrome (APS-1)
- Hereditary/Idiopathic - often presents in the first decade; may be associated with DiGeorge syndrome (absent thymus + parathyroids), alopecia, and candidiasis
- Infiltrative - iron deposition (hemochromatosis), copper (Wilson's disease), tumor or granuloma
- Toxic - high-dose radiation, asparaginase, ethiofos
2. Hypomagnesemia
- Very common and often overlooked
- Causes both impaired PTH release from parathyroids AND end-organ resistance to PTH
- Must be corrected for calcium replacement to be effective
3. Vitamin D Deficiency / Resistance
- Inadequate sunlight, malnutrition, malabsorption syndromes (celiac, Crohn's), liver disease (impaired 25-hydroxylation), chronic renal failure (impaired 1-alpha-hydroxylation)
- Drug-induced: anticonvulsants (phenytoin accelerates Vitamin D catabolism)
4. Chronic Renal Failure
- Impaired 1-alpha-hydroxylation of vitamin D → low calcitriol → poor intestinal Ca absorption
- Hyperphosphatemia drives calcium-phosphate precipitation, lowering ionized Ca
5. Pseudohypoparathyroidism (Albright Hereditary Osteodystrophy)
- PTH is elevated but end-organ resistance exists (G-protein defect in adenylate cyclase signaling)
- Classic phenotype: short stature, round face, short 4th metacarpals, subcutaneous calcifications
- Serum Ca low, phosphate high, PTH high - Henry's Clinical Diagnosis, p. 254
6. Acute Pancreatitis
- Calcium precipitates as calcium soaps in the retroperitoneum/abdominal cavity
- Glucagon-stimulated calcitonin release may contribute
7. Other Causes
| Category | Examples |
|---|---|
| Massive blood transfusion | Citrate chelates calcium (risk highest with liver failure, CHF, low-output states) |
| Toxins | Hydrofluoric acid, ethylene glycol (complex and chelate Ca) |
| Tumor lysis syndrome / Rhabdomyolysis | Hyperphosphatemia drives Ca-PO₄ precipitation |
| Sepsis / Burns / Critical illness | Hypoalbuminemia + direct mechanisms |
| Respiratory alkalosis / Hyperventilation | Shift of ionized Ca to bound form (anxiety, alcohol withdrawal) |
| Drugs | Bisphosphonates, calcitonin, glucagon, heparin, protamine, loop diuretics, foscarnet |
| Hungry bone syndrome | Post-parathyroidectomy rapid re-mineralization |
| HIV/AIDS | Multiple mechanisms |
- Rosen's Emergency Medicine, p. 2524-2525; Textbook of Family Medicine 9e
Clinical Features
Severity is driven by the ionized calcium level and the rate of fall. Alkalosis (primary or compensatory) worsens symptoms by increasing calcium-albumin binding.
Neuromuscular (most prominent)
- Perioral and digital paresthesias - earliest symptom
- Muscle cramps and tetany - carpopedal spasm
- Chvostek sign - tapping facial nerve anterior to the ear → ipsilateral facial muscle twitch (sensitive but not specific)
- Trousseau sign - inflate BP cuff 20 mmHg above systolic for 3 minutes → carpal spasm (more specific for hypocalcemia)
- Laryngospasm - life-threatening, causes stridor
- Bronchospasm - shortness of breath
- Generalized seizures
Cardiovascular
- QT prolongation on ECG - the most common ECG finding; can precipitate life-threatening arrhythmias
- Low-voltage T waves
- Cardiovascular collapse, hypotension, CHF, angina, syncope
- Patients with Ca < 8.95 mg/dL have a 2.3-fold higher risk of sudden cardiac arrest compared to those with Ca > 9.55 mg/dL - Rosen's Emergency Medicine
Neuropsychiatric
- Anxiety, confusion, dementia, psychosis
- Increased intracranial pressure with papilledema (in PHP)
- Extrapyramidal symptoms: choreoathetosis, dystonia
Chronic Signs
- Cataracts (lenticular)
- Poor dentition
- Dry skin, coarse hair, pruritus
- Basal ganglia calcifications (seen on CT head)
- Subcutaneous calcifications
Diagnostic Testing
Serum labs:
- Total serum calcium (with albumin for correction)
- Ionized calcium (preferred - not affected by albumin)
- Serum phosphate
- Magnesium
- PTH (intact) - key to differentiate etiology
- 25(OH)D and 1,25(OH)₂D
- BUN, creatinine, LFTs
- Urinary calcium/creatinine ratio
Interpretation guide (Ca/Phosphate/PTH/Vitamin D):
| Disorder | Ca | Phosphate | PTH | Notes |
|---|---|---|---|---|
| Hypoparathyroidism | Low | High | Low/absent | - |
| Pseudohypoparathyroidism | Low | High | High | PTH resistance |
| Vitamin D deficiency | Low | Low | High (2° hyperPTH) | 25(OH)D low |
| Chronic renal failure | Low | High | High (2° hyperPTH) | 1,25(OH)₂D low |
| Hypomagnesemia | Low | Variable | Low/inappropriately normal | - |
From Henry's Clinical Diagnosis, Table 16.1 & 16.2
ECG: Essential in all patients - look for QT prolongation, T wave changes, arrhythmias.
Management
Acute / Symptomatic Hypocalcemia
IV calcium is the treatment of choice for symptomatic hypocalcemia (tetany, seizures, laryngospasm, arrhythmias):
- Calcium gluconate 10 mL of 10% solution (1 g = 93 mg elemental Ca) IV over 10 minutes - preferred (less tissue necrosis if extravasation occurs)
- Calcium chloride 10 mL of 10% solution (1 g = 272 mg elemental Ca) - faster rise but caustic; use via central line
- Follow with continuous IV calcium infusion (e.g., calcium gluconate 1-2 mg/kg/h elemental Ca) until oral therapy can be started
- Monitor ECG continuously; recheck ionized calcium every 1-4 hours
Concurrent management:
- Correct hypomagnesemia first - calcium replacement will not work until Mg is repleted
- Manage hyperphosphatemia (phosphate binders if needed)
- Correct alkalosis
Chronic / Mild Asymptomatic Hypocalcemia
- Oral calcium (calcium carbonate 1-3 g/day elemental Ca in divided doses, taken with meals)
- Vitamin D supplementation:
- Cholecalciferol (D3) for nutritional deficiency
- Calcitriol (1,25(OH)₂D) for hypoparathyroidism or renal failure (bypasses 1-alpha-hydroxylation step)
- Thiazide diuretics may be useful in hypoparathyroid patients - they enhance renal calcium reabsorption (contrasting with loop diuretics which promote calciuresis)
- PTH replacement therapy (teriparatide/recombinant PTH) in select hypoparathyroid patients
Textbook of Family Medicine 9e; Rosen's Emergency Medicine
Special Situations
- Post-thyroidectomy: Monitor Ca every 6-12 hours; treat prophylactically if Ca < 7.5 mg/dL or symptomatic. Most cases are transient (parathyroid stunning rather than removal). - Recent meta-analysis (PMID: 40285398, 2025) evaluated intraoperative interventions to reduce post-thyroidectomy hypocalcemia.
- Neonatal hypocalcemia: Early onset (< 72 h) from maternal diabetes/prematurity; late onset (5-10 days) from high-phosphate feeds or hypomagnesemia
- Hypoalbuminemia: Always calculate corrected calcium; ionized Ca is normal - this is "pseudohypocalcemia" and does not require treatment
- Renal failure: Use activated vitamin D (calcitriol); watch for hyperphosphatemia
Key Mnemonics
VITAMIN D causes of hypocalcemia:
- Vitamin D deficiency
- Infiltrative disease (parathyroids)
- Total thyroidectomy / neck surgery
- Acute pancreatitis
- Magnesium deficiency
- Inherited (pseudohypoparathyroidism, DiGeorge)
- Neoplasm / tumor lysis
- Drugs / Dialysis
Sources
- Rosen's Emergency Medicine: Concepts and Clinical Practice, Chapter 114
- Textbook of Family Medicine, 9e, Chapter 35
- Henry's Clinical Diagnosis and Management by Laboratory Methods, Chapter 16
- Brenner and Rector's The Kidney, 2-Volume Set
- Recent evidence: Meta-analysis on post-thyroidectomy hypocalcemia prevention (PMID 40285398) - J Otolaryngol Head Neck Surg, 2025
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