Hypocalcemia

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Hypocalcemia

Definition:

  • Total serum calcium < 8.5 mg/dL

Calcium Physiology

1. Total vs. Ionized Calcium

Type% of total calciumNotes
Protein-bound (mostly albumin)~40%↓ in hypoproteinemia → factitious hypocalcemia (ionized Ca2+ normal)
Ionized calcium~45%Physiologically active; main regulator of PTH secretion; unaffected by albumin but affected by pH

2. pH Effects on Calcium and PTH

  • ↑ pH → ↑ protein binding → ↓ ionized Ca2+ → ↑ PTH

  • ↓ pH → ↓ protein binding → ↑ ionized Ca2+ → ↓ PTH

3. Physiological Roles

  • Stabilizes resting membrane potential

  • Neuromuscular excitability regulation

  • Bone mineralization

  • Coagulation, enzyme cofactor

Calcium Homeostasis

HormoneEffect on Ca2+Effect on PO4Mechanism
PTH↑↓↑ renal Ca2+ reabsorption, ↓ renal PO4 reabsorption, ↑ 1,25-(OH)2D → ↑ gut Ca2+ absorption, ↑ bone resorption via RANKL
Calcitriol (Vit D3)↑↑↑ intestinal Ca2+ & PO4 absorption, ↑ renal reabsorption, enhances bone mineralization
Calcitonin↓↓Inhibits bone resorption; keeps calcium in bones

Mnemonic:

  • PTH: Phosphate Trashing Hormone

  • Calcitonin: “Calci-bone-in!”



Etiology of Hypocalcemia

MechanismExamples / Notes
Low PTHPostsurgical hypoparathyroidism, autoimmune destruction, congenital (DiGeorge)
High PTH (secondary hyperparathyroidism)Vitamin D deficiency (malabsorption, liver/kidney disease, sunlight deficiency), CKD
PTH resistancePseudohypoparathyroidism
OtherMedications (loop diuretics, bisphosphonates, calcitonin), massive blood transfusions (citrate), hypomagnesemia, hyperventilation, osteoblastic metastases, renal tubular disorders

Key point: Most common causes → hypoparathyroidism or vitamin D deficiency.

Clinical Features

Acute/Neurological:

  • Tetany, carpopedal spasm

  • Paresthesias (perioral, extremities)

  • Chvostek sign: facial twitching

  • Trousseau sign: BP cuff-induced spasm

  • Seizures

Chronic:

  • Cataracts, dental abnormalities, dry skin, brittle nails



Diagnosis

Laboratory:

  • Confirm true hypocalcemia: total & ionized Ca2+

  • Serum intact PTH: first-line study

  • Phosphate, magnesium, creatinine, 25(OH) vitamin D

PTHAdditional FindingsLikely Condition
Low↑ PhosphateHypoparathyroidism
HighLow/normal phosphate, ↓ 25(OH)DVitamin D deficiency
High↑ PhosphatePseudohypoparathyroidism, CKD
Low↓ MagnesiumMalabsorption, alcoholism

ECG:

  • Prolonged QT interval




Treatment

  1. Acute/Symptomatic (tetany, seizures, Ca ≤ 7.5 mg/dL)

    • Continuous cardiac monitoring

    • IV calcium (beware in patients on digoxin → arrhythmia risk)

  2. Chronic/Asymptomatic

    • Oral calcium supplementation

    • Correct underlying disorder (vitamin D deficiency, hypomagnesemia, CKD)

  3. Medication adjustments

    • Stop loop diuretics if possible

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