Hypocalcemia

Introduction

Hypocalcemia is defined as low serum calcium levels (total Ca²⁺ < 8.5 mg/dL or ionized Ca²⁺ < 4.65 mg/dL). Calcium homeostasis is primarily regulated by parathyroid hormone (PTH), vitamin D, and the calcium-sensing receptor. The condition can result from various disorders affecting these regulatory mechanisms or other pathological processes.

Understanding Calcium Levels

Types of Calcium Measurement

1. Total Calcium (Normal: 8.5-10.5 mg/dL)
   • Includes both bound and free calcium
   • Affected by albumin levels
   • Most commonly measured
2. Ionized Calcium (Normal: 4.65-5.25 mg/dL)
   • Physiologically active form
   • Not affected by albumin
   • More accurate but less available

 

When to Correct for Albumin

• Important: Always correct total calcium when albumin is abnormal
• Why: 40% of calcium is bound to albumin
• When: Any condition affecting albumin (liver disease, malnutrition, nephrotic syndrome)

 

Calcium Correction Formula and Examples

• Corrected Ca = Measured Ca + 0.8 × (4.0 - albumin)
• Example 1: Low Albumin
  • Measured Ca: 7.8 mg/dL
  • Albumin: 2.5 g/dL
  • Calculation: 7.8 + 0.8 × (4.0 - 2.5) = 9.0 mg/dL
  • Interpretation: Normal calcium (Pseudohypocalcemia)
• Example 2: High Albumin
  • Measured Ca: 9.5 mg/dL
  • Albumin: 5.0 g/dL
  • Calculation: 9.5 + 0.8 × (4.0 - 5.0) = 8.7 mg/dL
  • Interpretation: Lower than measured

Causes and Classification

Understanding PTH's Role

PTH is the main regulator of calcium homeostasis. Causes of hypocalcemia can be categorized based on PTH levels:

 

Medication-Induced Hypocalcemia

Important medications to remember:

• Bisphosphonates: Inhibit bone resorption
• Loop diuretics: Increase calcium excretion
• Denosumab: Blocks bone resorption
• Anticonvulsants: Affect vitamin D metabolism
• Multiple transfusions: Citrate binding

Clinical Features

Severity Classification

 

Key Clinical Signs

1. Neuromuscular Signs
   • Chvostek's Sign
     • How to test: Tap facial nerve anterior to ear
     • Positive: Facial muscle twitching
     • Significance: 15% false positive in normal people
   • Trousseau's Sign
     • How to test: Inflate BP cuff above systolic for 3 minutes
     • Positive: Carpopedal spasm
     • More specific than Chvostek's
2. Cardiac Manifestations
   • QT prolongation (calculate corrected QT)
   • T wave changes
   • Heart blocks
   • Monitor closely if QTc > 500ms

Diagnostic Approach

Step-by-Step Evaluation

1. Confirm True Hypocalcemia
   • Check total calcium and albumin
   • Calculate corrected calcium
   • Consider ionized calcium if available
2. Assess Severity
   • Review symptoms
   • Check ECG
   • Evaluate for tetany
3. Determine Cause
   • Check PTH level
   • Basic metabolic panel
   • Magnesium level
   • Vitamin D if appropriate

Required Lab Tests

 

 

Treatment

Emergency Management (Ca < 7.0 mg/dL or Symptomatic)

• Immediate Actions
  • IV calcium gluconate 1-2g
  • Cardiac monitoring
  • Repeat calcium every 1-2 hours
  • Consider ICU admission
• Concurrent Steps
  • Check magnesium
  • Start vitamin D if needed
  • Identify and treat cause

 

Non-Emergency Management

• Oral Supplementation
  • Calcium carbonate 1-2g TID
  • Calcium citrate if on PPI
  • Vitamin D supplementation
• Monitoring
  • Check calcium weekly until stable
  • Monitor symptoms
  • Adjust dose based on levels

 

Special Situations

• Hypomagnesemia
  • Correct magnesium first
  • Then address calcium
• Chronic Kidney Disease
  • Use activated vitamin D
  • Monitor phosphate
  • Consider calcimimetics