Hypocalcaemia

Topic Overview

Summary

Hypocalcaemia is defined as serum corrected calcium below 2.1 mmol/L (or ionised calcium below 1.0 mmol/L) and represents a critical endocrine and electrolyte emergency when severe or symptomatic. [1] The condition results from disruption of the tightly regulated calcium homeostasis involving parathyroid hormone (PTH), vitamin D, and calcitonin. [2] Clinical manifestations range from asymptomatic biochemical abnormalities to life-threatening neuromuscular excitability (tetany, laryngospasm), cardiac complications (QT prolongation, torsades de pointes), and seizures. [3]

Common aetiologies include hypoparathyroidism (most frequently post-surgical following thyroidectomy or parathyroidectomy), vitamin D deficiency, chronic kidney disease-mineral bone disorder (CKD-MBD), hypomagnesaemia, hungry bone syndrome, acute pancreatitis, and massive transfusion with citrate toxicity. [4] Rare genetic causes include DiGeorge syndrome and pseudohypoparathyroidism. [5]

Emergency management of severe symptomatic hypocalcaemia requires immediate intravenous calcium gluconate administration with continuous cardiac monitoring. [6] Chronic management involves oral calcium supplementation, active vitamin D analogues (alfacalcidol or calcitriol), and treatment of underlying aetiologies. [7] Recognition and correction of concurrent hypomagnesaemia is essential, as hypocalcaemia remains refractory to treatment until magnesium is repleted. [8]

Key Facts

• Definition: Corrected calcium less than 2.1 mmol/L or ionised calcium less than 1.0 mmol/L [1]
• Critical threshold: Severe symptomatic hypocalcaemia typically occurs at corrected calcium less than 1.9 mmol/L [3]
• Most common cause: Post-surgical hypoparathyroidism (thyroidectomy, parathyroidectomy) [4]
• Classic signs: Chvostek's sign (facial twitch), Trousseau's sign (carpopedal spasm) [9]
• ECG finding: QT interval prolongation (corrected QTc >440 ms in men, >460 ms in women) [3]
• Emergency treatment: 10 mL 10% calcium gluconate IV over 10 minutes [6]
• Refractory hypocalcaemia: Always check and correct magnesium (target >0.75 mmol/L) [8]
• Chronic hypoparathyroidism: Requires lifelong alfacalcidol/calcitriol plus calcium supplements [7]

Clinical Pearls

Corrected calcium calculation: Corrected Ca (mmol/L) = measured Ca + 0.02 × (40 − albumin g/L). This formula accounts for the 40-50% of total calcium bound to albumin. [1]

Post-thyroidectomy monitoring: Check calcium and PTH at 1, 6, and 12 hours post-operatively. Calcium nadir typically occurs 24-48 hours post-surgery. [10]

Magnesium-calcium relationship: Hypomagnesaemia (less than 0.7 mmol/L) impairs both PTH secretion and PTH receptor responsiveness. Hypocalcaemia will not correct until magnesium is normalised. [8]

Hungry bone syndrome: Massive skeletal calcium uptake following parathyroidectomy for severe hyperparathyroidism. Requires aggressive calcium and vitamin D replacement for weeks to months. [11]

Alkalosis worsens symptoms: Respiratory or metabolic alkalosis increases calcium binding to albumin, reducing ionised calcium and precipitating symptomatic hypocalcaemia at higher total calcium levels. [12]

Citrate toxicity: Massive transfusion (>4 units rapidly) chelates calcium. Consider empirical calcium supplementation during massive transfusion protocols. [13]

Why This Matters Clinically

Severe hypocalcaemia represents a medical emergency with significant morbidity and mortality if not promptly recognised and treated. [3] Tetany and laryngospasm can precipitate acute airway compromise requiring emergency intubation. [14] Prolonged QT interval predisposes to polymorphic ventricular tachycardia (torsades de pointes) and sudden cardiac death. [3] Hypocalcaemic seizures may be refractory to standard anticonvulsant therapy and require calcium replacement for resolution. [15]

Post-operative hypocalcaemia following thyroid or parathyroid surgery occurs in 20-30% of patients and is the most common complication necessitating prolonged hospital admission. [10] Early recognition and protocol-driven management reduces hospital length of stay and prevents readmissions. [10]

Chronic hypoparathyroidism, affecting approximately 75,000 patients in the United States, significantly impairs quality of life and increases risk of renal complications (nephrocalcinosis, nephrolithiasis), cataracts, basal ganglia calcification, and neuropsychiatric symptoms. [7] Optimal long-term management requires endocrinology specialist input and regular monitoring to avoid oscillations between hypocalcaemia and iatrogenic hypercalciuria. [7]

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Visual Summary

Visual assets to be added:

• Calcium homeostasis pathway (PTH-vitamin D-calcitonin axis)
• Chvostek's sign demonstration (tapping facial nerve)
• Trousseau's sign demonstration (BP cuff inflation causing carpopedal spasm)
• ECG comparison: normal QT vs prolonged QT in hypocalcaemia
• Emergency management algorithm for severe symptomatic hypocalcaemia
• Hungry bone syndrome pathophysiology diagram
• Post-thyroidectomy monitoring protocol flowchart

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Epidemiology

Incidence and Prevalence

Hypocalcaemia prevalence varies widely depending on clinical setting and population studied:

Demographics

Age distribution:

• Neonatal: DiGeorge syndrome, maternal hyperparathyroidism, prematurity
• Childhood: Genetic hypoparathyroidism, vitamin D deficiency (rickets)
• Adults: Post-surgical hypoparathyroidism, vitamin D deficiency, CKD
• Elderly: Vitamin D deficiency (>50% in nursing homes), malabsorption, CKD [4]

Sex differences:

• Female predominance in autoimmune hypoparathyroidism (3:1 female:male ratio)
• Post-surgical hypoparathyroidism reflects thyroid surgery demographics (4:1 female:male)
• No sex difference in vitamin D deficiency or CKD-related hypocalcaemia [4]

Geographical variation:

• Higher vitamin D deficiency prevalence in Northern latitudes (reduced UVB exposure)
• Higher in populations with darker skin pigmentation
• Cultural factors: clothing coverage, indoor lifestyle [4]

Aetiology and Risk Factors

Exam Detail: #### Classification by Mechanism

1. Hypoparathyroidism (Low/Inappropriately Normal PTH)

2. Vitamin D Deficiency/Resistance (High PTH - Secondary Hyperparathyroidism)

3. Calcium Sequestration/Deposition (High PTH)

4. PTH Resistance (High PTH)

5. Chelation/Binding

6. Medications

Genetic Causes

DiGeorge Syndrome (22q11.2 deletion syndrome):

• Most common genetic cause of hypoparathyroidism
• Parathyroid aplasia/hypoplasia
• Associated cardiac defects, immune deficiency, characteristic facies
• Incidence 1:4,000 live births [5]

Autoimmune Polyglandular Syndrome Type 1 (APECED):

• AIRE gene mutations
• Triad: chronic mucocutaneous candidiasis, hypoparathyroidism, Addison's disease
• Autosomal recessive
• Presents childhood/adolescence [4]

Familial Isolated Hypoparathyroidism:

• PTH gene mutations, GCM2 mutations, CASR gain-of-function mutations
• Autosomal dominant, recessive, or X-linked inheritance patterns
• Highly variable penetrance and severity [5]

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Pathophysiology

Calcium Homeostasis: The PTH-Vitamin D Axis

Exam Detail: Normal calcium homeostasis maintains serum calcium within the narrow physiological range of 2.1-2.6 mmol/L through coordinated actions of parathyroid hormone (PTH), vitamin D (calcitriol), and calcitonin. [2]

Calcium distribution:

• Total body calcium: ~1,000-1,200 g (98% in skeleton, 1% intracellular, 1% extracellular)
• Serum total calcium: 2.1-2.6 mmol/L
  • "Ionised (free) calcium: 1.1-1.3 mmol/L (~50% of total) — physiologically active"
  • "Protein-bound: ~40% (primarily albumin)"
  • "Complexed with anions: ~10% (citrate, phosphate, bicarbonate) [1]"

Parathyroid hormone (PTH):

• 84 amino acid peptide secreted by chief cells of parathyroid glands
• Secretion inversely regulated by ionised calcium via calcium-sensing receptor (CaSR)
• Actions increase serum calcium:
  1. Bone: ↑ osteoclastic bone resorption (indirect via RANKL)
  2. Kidney: ↑ distal tubular calcium reabsorption, ↑ 1α-hydroxylase (activates vitamin D), ↓ phosphate reabsorption
  3. Intestine: ↑ absorption (indirect via vitamin D) [2]

Vitamin D metabolism:

• Vitamin D₃ (cholecalciferol) synthesised in skin (UVB) or dietary intake
• 25-hydroxylation in liver → 25(OH)D (calcidiol) — storage form, measured clinically
• 1α-hydroxylation in kidney → 1,25(OH)₂D (calcitriol) — active hormone
• Calcitriol actions:
  1. Intestine: ↑↑ calcium and phosphate absorption (primary action)
  2. Bone: ↑ mineralisation (with adequate calcium/phosphate)
  3. Parathyroid: ↓ PTH synthesis (negative feedback) [2]

Calcitonin:

• 32 amino acid peptide from thyroid C cells
• Weak hypocalcaemic effect (↓ osteoclast activity)
• Minimal role in normal calcium homeostasis; significant only in medullary thyroid cancer [2]

Mechanisms of Hypocalcaemia

1. PTH Deficiency (Hypoparathyroidism):

• ↓ Renal calcium reabsorption → ↑ urinary calcium loss
• ↓ Bone resorption → ↓ calcium release from skeleton
• ↓ 1α-hydroxylase activity → ↓ calcitriol → ↓ intestinal calcium absorption
• ↓ Renal phosphate excretion → hyperphosphataemia (characteristic biochemical feature)
• Net effect: ↓ serum calcium, ↑ serum phosphate, ↓/normal vitamin D [4]

2. Vitamin D Deficiency:

• ↓ Intestinal calcium absorption (primary defect)
• ↓ Serum calcium → ↑ PTH secretion (secondary hyperparathyroidism)
• ↑ PTH partially compensates via ↑ renal calcium reabsorption, ↑ bone resorption
• ↑ Renal phosphate excretion → hypophosphataemia (if prolonged)
• Net effect: ↓/normal serum calcium, ↓ phosphate, ↑ PTH, ↓ 25(OH)D [4]

3. PTH Resistance (Pseudohypoparathyroidism):

• Defective Gsα protein (GNAS gene mutations) impairs PTH receptor signaling
• Biochemically mimics hypoparathyroidism: ↓ calcium, ↑ phosphate
• But PTH is elevated (resistance, not deficiency)
• PHP1 a: Albright hereditary osteodystrophy phenotype (short stature, round facies, brachydactyly, subcutaneous ossifications, intellectual disability)
• PHP1 b: biochemical abnormalities without AHO phenotype [5]

4. Hungry Bone Syndrome:

• Occurs following parathyroidectomy in patients with severe prolonged hyperparathyroidism
• Pre-operative: high bone turnover with osteitis fibrosa cystica
• Post-operative: abrupt ↓ PTH → ↓ bone resorption, but osteoblastic bone formation continues
• Massive skeletal calcium and phosphate uptake (remineralisation)
• Severe, prolonged hypocalcaemia and hypophosphataemia
• May require weeks to months of aggressive replacement [11]

5. Hypomagnesaemia:

• Dual mechanism:
  1. Impaired PTH secretion (magnesium required for stimulus-secretion coupling)
  2. Skeletal resistance to PTH action
• Severe hypomagnesaemia (less than 0.4 mmol/L): functional hypoparathyroidism (low/normal PTH)
• Moderate hypomagnesaemia: PTH secretion may be elevated but ineffective
• Hypocalcaemia refractory to calcium replacement until magnesium corrected [8]

6. Acute Pancreatitis:

• Multiple mechanisms:
  1. Saponification: lipolysis releases fatty acids → calcium-soap complexes in peripancreatic fat necrosis
  2. Glucagon release → calcitonin secretion
  3. Hypoalbuminaemia (↓ total calcium, ionised calcium may be normal)
  4. Hypomagnesaemia (common in alcohol-related pancreatitis)
• Degree of hypocalcaemia correlates with severity (Ranson's criteria) [4]

Cellular Effects of Hypocalcaemia

Neuromuscular excitability:

• Ionised calcium stabilises neuronal membranes by blocking sodium channels
• Hypocalcaemia → ↓ threshold for action potential generation
• ↑ Neuronal and neuromuscular excitability
• Clinical: paraesthesia, tetany, carpopedal spasm, laryngospasm, seizures [3]

Cardiac effects:

• Calcium essential for phase 2 (plateau) of cardiac action potential
• Hypocalcaemia → prolonged plateau phase → QT interval prolongation
• Risk of torsades de pointes (polymorphic VT)
• Negative inotropic effect → heart failure (severe cases)
• ECG: prolonged QTc, T wave changes, rarely ventricular arrhythmias [3]

Smooth muscle effects:

• Bronchospasm, biliary colic, intestinal colic (rare)
• Laryngospasm (life-threatening airway emergency) [14]

Intracellular signalling:

• Calcium second messenger functions impaired
• Potential effects on insulin secretion, neurotransmitter release, enzyme function [2]

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Clinical Presentation

Symptoms

Symptom severity correlates with:

1. Absolute calcium level
2. Rate of decline (acute drop more symptomatic than chronic)
3. Concurrent acid-base status (alkalosis ↓ ionised calcium, worsens symptoms) [12]

Neuromuscular symptoms (most common):

• Paraesthesia: perioral, fingers, toes (often earliest symptom)
• Muscle cramps: hands, feet, legs
• Muscle stiffness and spasm
• Tetany: involuntary muscle contractions (severe)
• Carpopedal spasm: characteristic "obstetrician's hand" posture (severe)
• Laryngospasm: stridor, dyspnoea, acute airway obstruction (life-threatening) [3,14]

Central nervous system:

• Seizures: focal or generalised tonic-clonic (severe hypocalcaemia)
• Altered mental status: confusion, irritability, depression, anxiety
• Cognitive impairment: memory problems, difficulty concentrating
• Psychosis: rare, severe cases
• Papilloedema: raised intracranial pressure (chronic hypoparathyroidism) [15]

Cardiovascular:

• Palpitations
• Dyspnoea: heart failure (severe cases)
• Syncope: arrhythmia or seizure [3]

Chronic hypoparathyroidism (additional features):

• Dry skin, brittle nails, coarse hair
• Dental enamel hypoplasia (childhood onset)
• Cataracts (subcapsular)
• Fatigue, lethargy
• Neuropsychiatric symptoms: depression, anxiety, "brain fog" [7]

Signs

Neuromuscular examination:

Chvostek's sign:

• Technique: Tap facial nerve 2 cm anterior to ear lobe, below zygomatic arch
• Positive: Ipsilateral facial muscle twitch (orbicularis oculi, orbicularis oris, nasolabial)
• Sensitivity: 10-30% in hypocalcaemia (poor sensitivity)
• Specificity: 70-90% (present in 10-30% of normocalcaemic individuals)
• Interpretation: Positive sign supports hypocalcaemia but absence does not exclude [9]

Trousseau's sign:

• Technique: Inflate BP cuff on upper arm to 20 mmHg above systolic pressure for 3 minutes
• Positive: Carpopedal spasm ("main d'accoucheur" / obstetrician's hand)
  • Wrist and MCP flexion
  • PIP/DIP extension
  • Thumb adduction into palm
• Sensitivity: 40-70% in hypocalcaemia (more sensitive than Chvostek's)
• Specificity: 90-99% (highly specific)
• Interpretation: Most useful clinical sign for hypocalcaemia [9]

Other neuromuscular signs:

• Hyperreflexia
• Spontaneous tetany: without provocation (severe)
• Laryngospasm: stridor, inspiratory difficulty
• Seizure activity: focal or generalised [3]

Cardiovascular examination:

• Usually normal unless severe
• Signs of heart failure: elevated JVP, S3 gallop, pulmonary oedema (rare, severe cases)
• Arrhythmia: irregular pulse [3]

Chronic hypoparathyroidism findings:

• Dermatological: dry skin, eczema, psoriasis, fungal infections (APS-1), vitiligo (autoimmune)
• Ophthalmological: cataracts (posterior subcapsular)
• Dental: enamel hypoplasia, delayed tooth eruption (childhood onset)
• Skeletal: short stature (if childhood onset)
• Basal ganglia calcification: extrapyramidal signs (parkinsonism, dystonia) — rare [7]

Syndromic features:

• DiGeorge syndrome: hypertelorism, short philtrum, micrognathia, low-set ears, cardiac murmurs
• Albright hereditary osteodystrophy (PHP1a): round facies, short stature, brachydactyly (esp. 4th/5th metacarpals), subcutaneous ossifications, intellectual disability
• Autoimmune polyglandular syndrome 1: candidiasis (nails, oral), vitiligo, alopecia, hyperpigmentation (Addison's) [5]

Red Flags — Immediate Action Required

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Clinical Examination

History Taking — Key Questions

Exam Detail: Presenting complaint:

• "Have you noticed tingling or numbness, especially around your mouth or in your fingers?"
• "Have you had muscle cramps or spasms?"
• "Any episodes where your hands or feet cramped into unusual positions?"
• "Any breathing difficulties, especially a tight feeling in your throat?" (laryngospasm)
• "Have you had any seizures or blackouts?"
• "Palpitations or awareness of your heartbeat?"

Timing and onset:

• "When did symptoms start?"
• "Did they come on suddenly or gradually?"
• "Are symptoms constant or intermittent?"
• "Anything that makes them better or worse?" (hyperventilation worsens symptoms)

Surgical history:

• "Have you had any operations on your neck or thyroid?" (thyroidectomy, parathyroidectomy)
• "If so, when? Any complications?"

Dietary and nutritional:

• "Tell me about your diet — do you consume dairy products?"
• "Do you take any supplements — calcium or vitamin D?"
• "Any problems with your digestion or absorption?" (coeliac, IBD, bariatric surgery)
• "How much sunlight exposure do you get?"

Medical history:

• "Any kidney problems?"
• "Bone or joint problems?"
• "Autoimmune conditions?" (APS-1, thyroid disease)
• "Previous pancreatitis?"
• "Alcohol intake?"

Medications:

• "What medications do you take?" (bisphosphonates, denosumab, cinacalcet, anticonvulsants, PPIs, diuretics)

Family history:

• "Any family members with calcium, parathyroid, or thyroid problems?"
• "Any genetic conditions in the family?"

Systemic enquiry:

• Chronic symptoms: fatigue, depression, cognitive problems, cataracts
• APS-1 screen: candidiasis, Addison's symptoms, vitiligo

Systematic Examination

General inspection:

• Syndromic features: DiGeorge, AHO
• Neck scars: thyroidectomy/parathyroidectomy
• Signs of chronic illness

Neuromuscular examination:

1. Chvostek's sign: tap facial nerve, observe for facial twitch
2. Trousseau's sign: BP cuff test (3 minutes above systolic)
3. Muscle tone: may be increased
4. Reflexes: often brisk (hyperreflexia)
5. Spontaneous tetany or carpopedal spasm

Cardiovascular examination:

• Pulse: rate, rhythm (arrhythmia)
• Blood pressure
• JVP
• Heart sounds: signs of failure (rare)
• Peripheral oedema

Skin and appendages:

• Dry skin, eczema
• Candidiasis (APS-1)
• Vitiligo, alopecia (autoimmune)
• Nails: brittle

Eyes:

• Cataracts (slit-lamp examination)
• Papilloedema (fundoscopy in chronic cases)

Dental:

• Enamel hypoplasia (childhood onset hypoparathyroidism)

Hands:

• Brachydactyly (short 4th/5th metacarpals in AHO)
• Subcutaneous ossifications (AHO)

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Investigations

First-Line Investigations

Blood tests — acute setting:

Corrected Calcium Formula:

Corrected Ca (mmol/L) = measured Ca + 0.02 × (40 − albumin g/L)

Example: Measured Ca 1.8 mmol/L, albumin 30 g/L

• Corrected Ca = 1.8 + 0.02 × (40 − 30) = 1.8 + 0.2 = 2.0 mmol/L (hypocalcaemia confirmed)

Note: Ionised calcium measurement is gold standard (unaffected by albumin) but not always available. [1]

Diagnostic Algorithm Based on PTH and Phosphate

Exam Detail: ``` Hypocalcaemia confirmed (corrected Ca less than 2.1 mmol/L) ↓ Check PTH and Phosphate ↓ ┌─────────────────────────────────────────────┐ │ │ │ PTH LOW or INAPPROPRIATELY NORMAL │ PTH HIGH │ │ ├─────────────────┬───────────────────────────┤ │ Phosphate HIGH │ Phosphate LOW/NORMAL │ Phosphate varies │ │ │ │ HYPOPARATHYROIDISM: │ SECONDARY HYPERPARATHYROIDISM: │ - Post-surgical │ - Hypomagnesaemia │ - Vitamin D deficiency (phosphate ↓) │ - Autoimmune │ (check Mg) │ - CKD-MBD (phosphate ↑↑) │ - Genetic │ - Acute severe illness │ - Malabsorption │ - Infiltrative │ │ │ │ │ PTH HIGH + PHOSPHATE HIGH: │ │ - Pseudohypoparathyroidism (PTH resistance) │ │ - Advanced CKD └────────────────────────────────────────────┘

### Second-Line Investigations

**Further biochemical tests:**

| Test | Indication | Interpretation |
|------|------------|----------------|
| **1,25-dihydroxyvitamin D (calcitriol)** | If PTH elevated but 25(OH)D normal | Low: CKD (↓ 1α-hydroxylase); Vitamin D resistance |
| **24-hour urine calcium** | Chronic hypoparathyroidism | Hypercalciuria common (low PTH → ↓ renal Ca reabsorption) |
| **Parathyroid antibodies** | Suspected autoimmune hypoparathyroidism | Positive in autoimmune hypoparathyroidism |
| **Genetic testing** | Familial cases, syndromic features | GNAS (PHP), 22q11 FISH (DiGeorge), AIRE (APS-1), PTH gene |
| **Renal function tests** | All patients | Assess for CKD-MBD |

**Electrocardiogram (ECG):**

**Key findings in hypocalcaemia:**
- **QT interval prolongation**: ↑ QTc (corrected QT)
  - "Normal: QTc less than 440 ms (men), less than 460 ms (women)"
  - "Hypocalcaemia: QTc often 500-600 ms or higher"
  - "Measurement: QT prolongation primarily from prolonged ST segment (plateau phase)"
- **T wave changes**: T wave flattening or inversion
- **Arrhythmias** (rare): ventricular ectopics, torsades de pointes, VT/VF [3]

**Serial ECGs:**
- Monitor QTc during treatment
- QTc normalisation confirms response to therapy

**Imaging:**

| Modality | Indication | Findings |
|----------|------------|----------|
| **CT brain (non-contrast)** | Chronic hypoparathyroidism, neurological symptoms | Basal ganglia calcification |
| **Renal ultrasound** | Chronic hypoparathyroidism | Nephrocalcinosis, renal calculi |
| **DEXA scan** | Chronic hypoparathyroidism | ↑ Bone mineral density (low bone turnover) |
| **Slit-lamp examination** | Chronic hypoparathyroidism | Posterior subcapsular cataracts |

**Post-Thyroidectomy/Parathyroidectomy Monitoring Protocol:**

> **Exam Detail:** **Timing of calcium checks:**
- **Intra-operative**: PTH levels (rapid assay if available)
- **Post-operative**:
  - 1 hour post-op
  - 6 hours post-op  
  - 12 hours post-op
  - Then daily until stable or discharge
- **Calcium nadir**: typically 24-48 hours post-surgery [10]

**Indications for PTH measurement:**
- Immediate post-parathyroidectomy: confirm adequate resection (PTH should drop >50%)
- 6-12 hours post-thyroidectomy: if PTH less than 1.0 pmol/L, high risk permanent hypoparathyroidism

**Hungry bone syndrome prediction:**
- Pre-operative risk factors:
  - Severe hyperparathyroidism (PTH >200 ng/L)
  - Elevated alkaline phosphatase
  - Osteitis fibrosa cystica on imaging
  - Vitamin D deficiency
- Post-operative: profound hypocalcaemia and hypophosphataemia within 24-48h [11]


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## Differential Diagnosis

### Conditions Mimicking Hypocalcaemia

> **Exam Detail:** | Condition | Key Differentiating Features | Investigations |
|-----------|------------------------------|----------------|
| **Pseudohypocalcaemia** | Artefactual low calcium due to EDTA contamination; patient asymptomatic | Repeat sample (non-EDTA tube); ionised calcium normal |
| **Hyperventilation syndrome** | Paraesthesia, carpopedal spasm, alkalosis; total calcium normal | ABG: respiratory alkalosis; ionised calcium low (↑ protein binding); total calcium normal |
| **Hypomagnesaemia alone** | Paraesthesia, arrhythmia; may cause hypocalcaemia | Mg less than 0.7 mmol/L; calcium may be normal or low |
| **Functional tetany** | Anxiety, panic attack; all electrolytes normal | Calcium, magnesium, phosphate all normal |
| **Hypokalaemia** | Paraesthesia, weakness, arrhythmia | K less than 3.5 mmol/L; calcium normal |
| **Guillain-Barré syndrome** | Ascending weakness, paraesthesia | Ascending pattern; CSF albuminocytological dissociation |


### Differential by Clinical Context

**Post-surgical setting:**
1. **Transient hypoparathyroidism**: parathyroid oedema, resolves days-weeks
2. **Permanent hypoparathyroidism**: parathyroid removal/devascularisation
3. **Hungry bone syndrome**: post-parathyroidectomy for severe hyperparathyroidism
4. **Hypomagnesaemia**: post-operative losses

**Critical care setting:**
1. **Citrate toxicity**: massive transfusion, plasmapheresis
2. **Sepsis**: vitamin D deficiency, PTH resistance, hypoalbuminaemia
3. **Acute kidney injury**: ↓ calcitriol production
4. **Pancreatitis**: calcium sequestration
5. **Rhabdomyolysis**: calcium deposition in damaged muscle

**Chronic outpatient setting:**
1. **Vitamin D deficiency**: dietary, malabsorption, lack of sunlight
2. **CKD-MBD**: stages 3-5 CKD
3. **Hypomagnesaemia**: PPI use, alcohol, diarrhoea
4. **Malabsorption**: coeliac, IBD, bariatric surgery
5. **Medication-induced**: bisphosphonates, denosumab, anticonvulsants

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## Management

### Emergency Management — Severe Symptomatic Hypocalcaemia

**Definition of severe/symptomatic:**
- Corrected calcium less than 1.9 mmol/L, or
- Any degree of hypocalcaemia with: tetany, seizures, laryngospasm, arrhythmia, QTc >500 ms [6]

**Immediate actions:**

**1. Resuscitation and monitoring:**
- ABCDE assessment
- Secure airway (laryngospasm risk — call anaesthetics early)
- Continuous cardiac monitoring (ECG telemetry)
- Pulse oximetry
- IV access (large bore)
- Senior support (medical/ICU registrar)

**2. IV calcium replacement (FIRST-LINE):**

**Initial bolus:**
- **10 mL of 10% calcium gluconate IV** (equivalent to 2.25 mmol elemental calcium)
- Administer over **10 minutes** (slow push)
- Requires cardiac monitoring during administration
- May repeat bolus if symptoms persist (up to 3 doses)

**CAUTION:**
- Do NOT give calcium chloride peripherally (severe extravasation risk; reserve for cardiac arrest via central line)
- Calcium gluconate is safer for peripheral administration
- Flush line well; do not mix with bicarbonate or phosphate (precipitation)

**Continuous infusion (if ongoing symptomatic hypocalcaemia):**
- **50-100 mL of 10% calcium gluconate in 500 mL-1 L of 0.9% saline or 5% dextrose**
- Infuse over 12-24 hours
- Titrate to maintain corrected calcium >1.9 mmol/L
- Check calcium levels 4-6 hourly during infusion
- Requires continuous cardiac monitoring [6]

**3. Magnesium replacement (ESSENTIAL):**

**Check magnesium urgently** — hypocalcaemia is refractory until magnesium corrected. [8]

If **Mg less than 0.7 mmol/L**:
- **20 mmol magnesium sulphate (8 mmol Mg²⁺) IV** in 100 mL 0.9% saline over 1 hour
- Repeat Mg level after 6 hours
- Further replacement as needed (target Mg >0.75 mmol/L)
- Maintenance: oral magnesium supplements

**4. Treat underlying cause:**
- Post-surgical: ensure adequate ongoing calcium/vitamin D replacement
- Pancreatitis: supportive care, fluid resuscitation
- CKD: nephrology input, dialysis consideration

**5. Monitoring:**
- Corrected calcium: 4-6 hourly initially, then 12-24 hourly once stable
- Magnesium: 6-12 hourly until normal
- Phosphate: 12-24 hourly
- ECG: continuous telemetry; repeat 12-lead ECG daily
- Clinical: neuromuscular signs (Trousseau's, Chvostek's)

**6. Transition to oral therapy:**
- Once stable on IV calcium (corrected Ca >2.0 mmol/L, asymptomatic):
  - Start oral calcium carbonate 1-2 g elemental calcium daily in divided doses
  - Start alfacalcidol or calcitriol (see chronic management)
  - Wean IV calcium gradually over 24-48 hours
  - Monitor calcium levels closely during transition

> **Exam Detail:** **Calcium Gluconate vs Calcium Chloride:**

| Feature | Calcium Gluconate 10% | Calcium Chloride 10% |
|---------|----------------------|----------------------|
| **Elemental calcium per 10 mL** | 2.25 mmol (90 mg) | 6.8 mmol (272 mg) |
| **Route** | IV (peripheral or central) | IV (central only) |
| **Extravasation risk** | Moderate | SEVERE (tissue necrosis) |
| **Indications** | Hypocalcaemia treatment (first-line) | Cardiac arrest (ACLS), central line only |
| **Onset** | Immediate | Immediate |
| **Metabolism** | Requires liver metabolism | Directly ionised |

**Recommendation**: Calcium gluconate is preferred for hypocalcaemia management due to lower extravasation risk. Calcium chloride reserved for cardiac arrest or when central access available. [6]


### Chronic Hypocalcaemia Management

**Mild-moderate hypocalcaemia (corrected Ca 1.9-2.1 mmol/L, asymptomatic):**

**Oral calcium supplementation:**
- **Calcium carbonate**: 1-2 g elemental calcium daily in divided doses (e.g., 500 mg TDS-QDS)
  - Take with food for optimal absorption
  - Provides 40% elemental calcium by weight
- **Calcium citrate**: alternative if gastric pH issues (achlorhydria, PPI use)
  - Better absorbed in low-acid environment
  - Provides 21% elemental calcium by weight
  - Can take without food

**Vitamin D supplementation:**

**If vitamin D deficiency (25-OH vitamin D less than 30 nmol/L):**
- **Loading**: Cholecalciferol (vitamin D₃) 20,000-40,000 units weekly for 8-12 weeks
- **Maintenance**: 800-2,000 units daily
- Recheck 25-OH vitamin D after 3 months [4]

**If hypoparathyroidism:**
- **Active vitamin D analogue required** (cannot rely on renal 1α-hydroxylase)
- **Alfacalcidol**: 0.25-1 mcg daily (typical starting dose 0.5 mcg)
  - Requires 25-hydroxylation in liver (single step)
- **Calcitriol**: 0.25-0.5 mcg daily
  - Fully active (1,25-dihydroxyvitamin D)
  - Shorter half-life than alfacalcidol
- Titrate dose based on calcium levels
- Target: serum calcium in low-normal range (2.0-2.2 mmol/L) to avoid hypercalciuria [7]

**Monitoring in chronic hypoparathyroidism:**
- Serum calcium, phosphate: every 3-6 months once stable
- 24-hour urine calcium: annually (monitor for hypercalciuria)
- Renal function: annually
- Renal ultrasound: every 2-5 years (nephrocalcinosis surveillance)
- Ophthalmology: baseline and every 2 years (cataract screening)
- DEXA scan: baseline and every 3-5 years [7]

**Complications of over-treatment:**
- **Hypercalciuria**: ↑ risk nephrolithiasis, nephrocalcinosis (low PTH → ↓ renal calcium reabsorption)
- **Hypercalcaemia**: symptoms, renal impairment
- Strategy: aim for low-normal calcium (2.0-2.2 mmol/L) rather than mid-normal

### Special Situations

**Hungry Bone Syndrome:**
- Occurs post-parathyroidectomy in severe hyperparathyroidism
- **Anticipate and prevent**:
  - Pre-operative vitamin D repletion (ideally 25-OH vitamin D >50 nmol/L)
  - Start calcium and calcitriol immediately post-operatively
- **Treatment**:
  - "High-dose oral calcium: 3-6 g elemental calcium daily"
  - Calcitriol 1-2 mcg daily (or alfacalcidol 1-3 mcg daily)
  - May require IV calcium infusion for several days
  - Monitor calcium and phosphate 6-12 hourly initially
  - "Duration: weeks to months (median 2-3 months)"
  - Gradual weaning as bone remineralisation completes [11]

**CKD-Mineral Bone Disorder (CKD-MBD):**
- Complex disorder: hypocalcaemia, hyperphosphataemia, secondary hyperparathyroidism, vitamin D deficiency
- **Management**:
  - "Phosphate restriction: dietary + phosphate binders (calcium carbonate, sevelamer, lanthanum)"
  - "Active vitamin D: alfacalcidol or calcitriol (cautious — may worsen hyperphosphataemia)"
  - "Calcimimetics: cinacalcet (if severe secondary hyperparathyroidism)"
  - "Target: calcium 2.1-2.4 mmol/L, phosphate less than 1.8 mmol/L, PTH 2-9× upper limit normal (CKD stage dependent)"
  - Nephrology co-management essential [16]

**Pregnancy and Hypoparathyroidism:**
- Calcium requirements increase in pregnancy
- Monitor calcium monthly (first trimester), then 2-weekly (second/third trimester)
- May need increased calcium and vitamin D doses
- Calcitriol preferred over alfacalcidol (less placental transfer)
- Avoid excessive vitamin D (fetal hypercalcaemia risk)
- Post-partum: calcium requirements decrease (risk hypercalcaemia — reduce doses)
- Multidisciplinary care: endocrinology, obstetrics [7]

**Pseudohypoparathyroidism (PHP):**
- Management similar to hypoparathyroidism (calcium and active vitamin D)
- PHP1
a: additional hormone resistance (TSH, gonadotropins) — may need levothyroxine, sex hormones
- Genetic counselling
- Monitor for complications: subcutaneous ossifications, obesity [5]

**Autoimmune Polyglandular Syndrome Type 1 (APS-1):**
- Screen for other autoimmune manifestations:
  - Addison's disease (annual cortisol, ACTH, renin)
  - Hypothyroidism (annual TSH, T4)
  - Diabetes (annual glucose, HbA1c)
  - Gonadal failure (FSH, LH, oestradiol/testosterone)
  - Pernicious anaemia (FBC, B12)
  - Chronic candidiasis (antifungal prophylaxis)
- Endocrinology long-term follow-up [4]

> **Exam Detail:** ### Recombinant Human PTH(1-84) — Specialist Therapy

**Indication:**
- Chronic hypoparathyroidism inadequately controlled on conventional therapy (calcium + vitamin D)
- Recurrent hypocalcaemia or hypercalciuria
- Approved in USA and Europe (not routinely available NHS)

**Mechanism:**
- Subcutaneous recombinant PTH(1-84) injection once daily
- Restores physiological PTH action: ↑ renal calcium reabsorption, ↑ 1α-hydroxylase, ↑ bone turnover

**Benefits:**
- ↓ Calcium and vitamin D requirements
- ↓ Hypercalciuria (↑ renal calcium reabsorption)
- Improved quality of life

**Limitations:**
- High cost
- Daily injections
- Risk hypercalcaemia
- Long-term safety data limited [7]


---

## Complications

### Complications of Untreated Hypocalcaemia

**Acute life-threatening:**
- **Laryngospasm**: airway obstruction, asphyxia [14]
- **Seizures**: status epilepticus, cerebral injury [15]
- **Torsades de pointes**: polymorphic VT, cardiac arrest [3]
- **Heart failure**: negative inotropy (severe cases)

**Chronic complications:**
- **Cataracts**: posterior subcapsular (commonest; 40-50% chronic hypoparathyroidism)
- **Basal ganglia calcification**: extrapyramidal features (parkinsonism, dystonia, chorea)
- **Cognitive impairment**: memory problems, "brain fog," reduced quality of life
- **Neuropsychiatric**: depression, anxiety
- **Dental**: enamel hypoplasia (childhood onset)
- **Dermatological**: dry skin, eczema, brittle nails, coarse hair
- **Papilloedema**: raised ICP (mechanism unclear)
- **Skeletal**: ↑ bone mineral density (paradoxically — low bone turnover) [7]

### Complications of Treatment

**Hypercalcaemia:**
- Over-treatment with calcium and/or vitamin D
- Symptoms: polyuria, polydipsia, constipation, confusion
- Management: reduce/stop calcium and vitamin D; IV fluids if severe

**Hypercalciuria:**
- Low PTH → ↓ renal calcium reabsorption
- Even when serum calcium low-normal, urinary calcium may be elevated
- Complications: nephrolithiasis, nephrocalcinosis
- Monitoring: annual 24-hour urine calcium (less than 7.5 mmol/24h target)
- Management: thiazide diuretic (↑ renal calcium reabsorption); reduce calcium/vitamin D doses [7]

**Nephrocalcinosis and chronic kidney disease:**
- Long-term complication (10-30% chronic hypoparathyroidism)
- Due to hypercalciuria and hyperphosphataemia (calcium-phosphate product)
- Monitor: annual renal function, periodic renal ultrasound

**Extravasation injury:**
- IV calcium gluconate extravasation → tissue necrosis (especially calcium chloride)
- Prevention: secure IV access, regular inspection, use calcium gluconate not chloride peripherally

**Vitamin D toxicity:**
- Excessive vitamin D (rare with physiological doses)
- Hypercalcaemia, hypercalciuria
- Long half-life (weeks) — takes time to resolve

---

## Prognosis and Long-Term Outcomes

### Prognosis by Cause

**Post-surgical hypoparathyroidism:**
- **Transient** (60-80% of cases): Resolves within 6 months; parathyroid oedema/stunning
- **Permanent** (1-2% post-thyroidectomy, 10-30% post-parathyroidectomy): Lifelong treatment required
- Prognosis with treatment: excellent
- Quality of life: reduced compared to general population, even with optimal biochemical control [10]

**Vitamin D deficiency:**
- Excellent prognosis with repletion
- Symptoms resolve over weeks to months
- Address underlying cause (dietary, malabsorption)

**CKD-MBD:**
- Prognosis depends on CKD stage and progression
- Optimal management reduces cardiovascular morbidity
- Renal replacement therapy (dialysis) may be required [16]

**Autoimmune hypoparathyroidism:**
- Chronic, lifelong condition
- Monitor for other autoimmune manifestations (APS-1)
- Good prognosis with treatment adherence

**Genetic causes:**
- Lifelong requirement for therapy
- Genetic counselling for family planning
- Prognosis good with treatment

### Quality of Life in Chronic Hypoparathyroidism

Multiple studies demonstrate **reduced quality of life** in chronic hypoparathyroidism patients compared to general population, even when biochemical control achieved:
- Physical: fatigue, muscle weakness
- Cognitive: "brain fog," memory problems, concentration difficulties
- Psychological: depression, anxiety
- Treatment burden: multiple daily medications, frequent monitoring [7]

**Factors associated with better outcomes:**
- Stable biochemical control
- Avoidance of hypocalcaemic episodes
- Endocrinology specialist follow-up
- Patient education and self-management

### Long-Term Monitoring

**For all chronic hypoparathyroidism patients:**

| Test | Frequency | Target/Purpose |
|------|-----------|----------------|
| Serum calcium, phosphate | 3-6 monthly once stable | Calcium 2.0-2.2 mmol/L (low-normal) |
| Serum magnesium | Annually | >0.75 mmol/L |
| Renal function (eGFR) | Annually | Monitor for CKD |
| 24-hour urine calcium | Annually | less than 7.5 mmol/24h (avoid nephrolithiasis) |
| Renal ultrasound | Every 2-5 years | Screen nephrocalcinosis |
| Ophthalmology review | Baseline, then every 2 years | Cataract screening |
| DEXA bone density | Baseline, then every 3-5 years | Paradoxically high BMD (low turnover) |

**Red flags for specialist referral:**
- Recurrent hypocalcaemia despite treatment
- Hypercalciuria (>7.5 mmol/24h)
- Nephrocalcinosis or nephrolithiasis
- Declining renal function
- Poor quality of life despite optimal biochemical control
- Pregnancy or planning pregnancy
- Paediatric cases

---

## Prevention

### Primary Prevention

**Post-surgical hypoparathyroidism prevention:**
- Careful surgical technique (thyroidectomy, parathyroidectomy)
- Identification and preservation of parathyroid glands
- Vascular preservation (parathyroid blood supply)
- Consider auto-transplantation of parathyroid tissue if devascularised
- Surgeon experience and case volume important [10]

**Vitamin D deficiency prevention:**
- Adequate sunlight exposure (15-20 minutes daily, arms/face)
- Dietary vitamin D: oily fish, fortified foods
- Supplementation in high-risk groups: elderly, housebound, dark skin, pregnancy
- Public health: vitamin D fortification programmes [4]

**Hypomagnesaemia prevention:**
- Dietary magnesium: green vegetables, nuts, whole grains
- Correct hypomagnesaemia in at-risk populations: alcohol excess, chronic diarrhoea, PPI users, diuretics

### Secondary Prevention

**Post-thyroidectomy/parathyroidectomy protocol:**
- **Pre-operative**:
  - Vitamin D repletion (target 25-OH vitamin D >50 nmol/L)
  - Calcium supplementation if deficient
- **Intra-operative**:
  - PTH monitoring (rapid assay where available)
- **Post-operative**:
  - Protocol-driven calcium and PTH monitoring (1h, 6h, 12h, 24h, 48h)
  - Pre-emptive calcium and vitamin D supplementation in high-risk patients
  - Early discharge if calcium stable >2.0 mmol/L at 24-48h [10]

**Hungry bone syndrome prevention:**
- Pre-operative vitamin D optimisation
- Early post-operative calcium and calcitriol (do not wait for hypocalcaemia)
- Anticipate in high-risk patients (severe hyperparathyroidism, elevated ALP, bone disease) [11]

**CKD-MBD management:**
- Early vitamin D monitoring and repletion in CKD stages 3-4
- Phosphate management (dietary restriction, binders)
- Nephrology co-management
- Prevent progression to severe secondary hyperparathyroidism [16]

---

## Evidence and Guidelines

### Key Guidelines

**1. Endocrine Society Clinical Practice Guideline: Hypoparathyroidism in Adults (2022)**
- Comprehensive management of chronic hypoparathyroidism
- Calcium and active vitamin D dosing
- Monitoring recommendations
- Recombinant PTH(1-84) therapy indications [7]

**2. Society for Endocrinology Emergency Guidance: Emergency Management of Acute Hypocalcaemia (2020)**
- IV calcium gluconate protocols
- Magnesium replacement
- Monitoring during acute management [6]

**3. European Society of Endocrinology Clinical Guideline: Treatment of Chronic Hypoparathyroidism (2015)**
- Long-term management strategies
- Quality of life considerations
- Surveillance for complications [7]

**4. KDIGO Guidelines: CKD-Mineral and Bone Disorder (2017)**
- CKD-MBD management
- Calcium, phosphate, and PTH targets
- Vitamin D supplementation in CKD [16]

**5. British Association of Endocrine and Thyroid Surgeons: Post-Thyroidectomy Hypocalcaemia Management (2021)**
- Post-operative monitoring protocols
- Calcium replacement algorithms
- Discharge criteria [10]

### Key Evidence

**Post-surgical hypoparathyroidism:**
- Asari et al. (2008): Post-thyroidectomy hypocalcaemia incidence and risk factors. [10]
- Bollerslev et al. (2015): European multicentre study on chronic hypoparathyroidism quality of life and management. [7]

**Emergency management:**
- Fong & Khan (2012): Hypocalcaemia: updates in diagnosis and management. [6]
- Cooper & Gittoes (2008): Diagnosis and management of hypocalcaemia (BMJ clinical review). [1]

**Hungry bone syndrome:**
- Witteveen et al. (2013): Hungry bone syndrome after parathyroidectomy: systematic review. [11]

**Vitamin D and calcium homeostasis:**
- Holick (2007): Vitamin D deficiency (NEJM seminal review). [4]
- Christakos et al. (2016): Vitamin D: molecular mechanism of action. [2]

**PTH physiology:**
- Shoback (2008): Hypoparathyroidism (NEJM). [4]
- Bilezikian et al. (2011): Hypoparathyroidism in the adult: epidemiology, diagnosis, pathophysiology, target-organ involvement, treatment. [7]

**Magnesium-calcium relationship:**
- Rude & Singer (1981): Magnesium deficiency and excess (NEJM). [8]
- Fong & Khan (2012): Hypocalcaemia and hypomagnesaemia. [8]

---

## Examination Focus

### MRCP PACES Station 5 — Integrated Clinical Assessment

> **Exam Detail:** **Scenario:**
"This 52-year-old woman presents to the Emergency Department with tingling in her hands and around her mouth for the past 2 days. She has a past medical history of thyrotoxicosis. Please assess her and suggest a management plan."

**Expected approach:**

**History:**
- Presenting complaint: perioral and digital paraesthesia, progression
- Neuromuscular symptoms: muscle cramps, spasms, carpopedal spasm
- Red flags: tetany, breathing difficulty (laryngospasm), seizures, palpitations
- Past surgical history: thyroidectomy (when? complications?)
- Medications: calcium, vitamin D, other
- Dietary calcium and vitamin D intake
- Other causes: CKD, pancreatitis, alcohol

**Examination:**
- General: neck scar (thyroidectomy)
- Neuromuscular: Chvostek's sign, Trousseau's sign (demonstrate technique)
- Cardiovascular: arrhythmia

**Investigations:**
- "I would check corrected serum calcium, albumin, magnesium, phosphate, PTH, vitamin D, renal function, and ECG."

**Differential diagnosis:**
- "Most likely post-thyroidectomy hypoparathyroidism given surgical history and timing."
- "Differentials: vitamin D deficiency, hypomagnesaemia, CKD."

**Management:**
- "If severe/symptomatic (Ca less than 1.9 or tetany): IV calcium gluconate 10 mL 10% over 10 minutes with cardiac monitoring."
- "Check and correct magnesium."
- "Chronic management: oral calcium carbonate plus alfacalcidol or calcitriol."
- "Endocrinology referral for long-term management."

**Examiner questions:**
1. "What is Trousseau's sign and how do you elicit it?"
   - "Inflate BP cuff to 20 mmHg above systolic for 3 minutes. Positive if carpopedal spasm occurs — wrist and MCP flexion, finger extension, thumb adduction."

2. "How do you correct calcium for albumin?"
   - "Corrected calcium = measured calcium + 0.02 × (40 − albumin in g/L)."

3. "Why is it important to check magnesium?"
   - "Hypomagnesaemia impairs PTH secretion and causes end-organ PTH resistance. Hypocalcaemia will not correct until magnesium is replaced."

4. "What ECG finding would you expect?"
   - "Prolonged QT interval, specifically prolonged ST segment. Risk of torsades de pointes if QTc >500 ms."

5. "What are the causes of hypocalcaemia with high phosphate?"
   - "Hypoparathyroidism (low PTH), pseudohypoparathyroidism (high PTH but resistance), advanced CKD."


### Viva Questions and Model Answers

> **Exam Detail:** **Q1: What is hungry bone syndrome?**

**Model answer:**
"Hungry bone syndrome is severe, prolonged hypocalcaemia and hypophosphataemia occurring after parathyroidectomy in patients with severe hyperparathyroidism. Pre-operatively, these patients have high bone turnover with increased osteoclastic and osteoblastic activity. Post-operatively, when PTH levels drop abruptly, osteoclastic bone resorption ceases but osteoblastic bone formation continues. This results in massive skeletal uptake of calcium and phosphate for bone remineralisation. Clinically, it manifests as profound hypocalcaemia within 24-48 hours post-surgery that may persist for weeks to months. Management requires aggressive calcium and calcitriol replacement, often necessitating IV calcium infusions initially. Risk factors include severe pre-operative hyperparathyroidism with elevated alkaline phosphatase and evidence of osteitis fibrosa cystica on imaging."

**Q2: Why is alfacalcidol or calcitriol used in hypoparathyroidism rather than cholecalciferol?**

**Model answer:**
"In hypoparathyroidism, PTH deficiency impairs renal 1α-hydroxylase enzyme activity, which is required to convert 25-hydroxyvitamin D (calcidiol) to 1,25-dihydroxyvitamin D (calcitriol), the active form. Cholecalciferol (vitamin D3) requires both hepatic 25-hydroxylation and renal 1α-hydroxylation to become active. Therefore, in hypoparathyroidism, giving cholecalciferol alone is ineffective. Alfacalcidol and calcitriol are active vitamin D analogues that bypass the need for renal 1α-hydroxylase. Alfacalcidol requires only hepatic 25-hydroxylation (one step), while calcitriol is already fully active. Both can effectively increase intestinal calcium absorption in hypoparathyroid patients. Doses are titrated to maintain serum calcium in the low-normal range (2.0-2.2 mmol/L) to minimise hypercalciuria."

**Q3: Describe the pathophysiology of QT prolongation in hypocalcaemia.**

**Model answer:**
"Calcium ions are essential for phase 2 (plateau phase) of the cardiac action potential. During this phase, calcium influx through L-type calcium channels maintains membrane depolarisation. In hypocalcaemia, reduced extracellular calcium prolongs the plateau phase as the cell attempts to maintain calcium influx. This manifests on the ECG as prolongation of the ST segment and overall QT interval. The prolonged QT interval increases the risk of early after-depolarisations, which can trigger polymorphic ventricular tachycardia known as torsades de pointes. This arrhythmia is potentially life-threatening and can degenerate into ventricular fibrillation and cardiac arrest. Other factors that prolong QT (hypomagnesaemia, hypokalaemia, QT-prolonging drugs) synergistically increase this risk. QTc prolongation >500 ms is particularly concerning and warrants urgent calcium replacement and continuous cardiac monitoring."

**Q4: What is pseudohypoparathyroidism and how does it differ from hypoparathyroidism?**

**Model answer:**
"Pseudohypoparathyroidism (PHP) is a group of rare genetic disorders characterised by resistance to PTH action rather than PTH deficiency. The biochemical picture mimics hypoparathyroidism — hypocalcaemia and hyperphosphataemia — but PTH levels are elevated, not low, due to end-organ resistance. PHP1a is caused by GNAS gene mutations affecting the Gsα subunit of the PTH receptor signalling pathway. Patients with PHP1a exhibit Albright hereditary osteodystrophy (AHO) phenotype: round facies, short stature, brachydactyly (especially 4th and 5th metacarpals), subcutaneous ossifications, and intellectual disability. PHP1b has the biochemical abnormalities but lacks the AHO phenotype. Management is similar to hypoparathyroidism — calcium and active vitamin D supplementation — but patients may also have resistance to other G-protein coupled receptor hormones like TSH and gonadotropins, requiring thyroid hormone replacement and sex hormone therapy. Genetic counselling is important as inheritance patterns vary (autosomal dominant with imprinting effects)."

**Q5: A patient post-thyroidectomy has corrected calcium 2.3 mmol/L but symptomatic with paraesthesia. What is your explanation and management?**

**Model answer:**
"This scenario suggests symptomatic hypocalcaemia despite apparently normal total calcium, which can occur in several situations. First, I would check ionised calcium, as this is the physiologically active fraction. Ionised calcium can be low even when total corrected calcium appears normal, particularly in the context of acid-base disturbances. Alkalosis increases calcium binding to albumin, reducing ionised calcium. Post-operative patients may develop respiratory alkalosis from anxiety or pain, precipitating symptoms. Second, rapid decline in calcium — even if not reaching absolute low levels — can cause symptoms as the neuromuscular system has not had time to adapt. Third, concurrent hypomagnesaemia can cause similar symptoms and should be checked and corrected. Management involves measuring ionised calcium and arterial blood gas, checking magnesium, and considering empirical calcium supplementation (oral initially given the calcium level, or IV if severe symptoms). I would also reassure the patient to reduce hyperventilation if present, and provide analgesia if pain is contributing to anxiety and tachypnoea."


---

## Patient and Family Information

### What is Hypocalcaemia?

Hypocalcaemia means you have low levels of calcium in your blood. Calcium is a mineral that is essential for many body functions, including:
- Keeping your bones and teeth strong
- Helping your muscles work properly
- Allowing your nerves to send messages
- Keeping your heart beating regularly

When calcium levels are too low, your nerves and muscles become overactive, causing symptoms like tingling, muscle cramps, and spasms.

### What Causes Low Calcium?

Common causes include:
- **Surgery on your thyroid or parathyroid glands** — These small glands in your neck help control calcium levels. Surgery can temporarily or permanently affect them.
- **Low vitamin D** — Your body needs vitamin D to absorb calcium from food. Not getting enough sunlight or having a poor diet can lead to low vitamin D.
- **Kidney problems** — Your kidneys help activate vitamin D and control calcium balance.
- **Low magnesium** — Magnesium is needed for calcium control. Low magnesium can cause low calcium.
- **Some medications** — Such as treatments for osteoporosis or kidney disease.

### What are the Symptoms?

Symptoms can include:
- **Tingling** or "pins and needles" around your mouth, fingers, or toes
- **Muscle cramps**, especially in your hands and feet
- **Muscle spasms** that you can't control
- **Anxiety** or feeling irritable
- **Confusion** or memory problems
- **Seizures** or blackouts (in severe cases)
- **Abnormal heart rhythms** (palpitations)

### How is it Diagnosed?

Your doctor will do a blood test to measure your calcium level. They may also check:
- Albumin (a protein in your blood that affects calcium levels)
- Magnesium
- Vitamin D
- Parathyroid hormone (PTH)
- Your kidney function
- An ECG (heart trace) to check your heart rhythm

### How is it Treated?

**Emergency treatment (if severe):**
- Calcium given through a drip into your vein in hospital
- Close monitoring of your heart and calcium levels

**Long-term treatment:**
- **Calcium tablets** — Usually calcium carbonate or calcium citrate, taken 2-4 times a day with meals
- **Vitamin D tablets** — Either regular vitamin D (cholecalciferol) or active vitamin D (alfacalcidol or calcitriol) depending on the cause
- **Magnesium tablets** — If your magnesium is also low
- **Treating the underlying cause** — For example, vitamin D for deficiency or lifelong treatment if your parathyroid glands were removed

### Living with Low Calcium

If you need long-term treatment:
- **Take your medications regularly** — It's important not to miss doses
- **Eat a calcium-rich diet** — Dairy products (milk, cheese, yoghurt), green leafy vegetables, nuts, and fortified foods
- **Get some sunlight** — 15-20 minutes a day helps your body make vitamin D
- **Regular blood tests** — To check your calcium levels and adjust treatment
- **Watch for symptoms** — Contact your doctor if you get tingling, cramps, or muscle spasms

### When to Seek Help

**Call 999 or go to A&E if you have:**
- Severe muscle spasms or hands/feet cramping into unusual positions
- Difficulty breathing or a tight throat
- Seizures or blackouts
- Severe confusion

**Contact your doctor if you have:**
- Tingling or numbness that doesn't go away
- Muscle cramps or weakness
- Palpitations or irregular heartbeat
- Signs your calcium is too high: excessive thirst, passing lots of urine, constipation, confusion

### Resources and Support

- **Parathyroid UK**: www.parathyroiduk.org — Patient support and information for parathyroid conditions
- **National Osteoporosis Society**: Information on calcium and vitamin D
- **NHS website**: www.nhs.uk — Search "hypocalcaemia" or "low calcium"
- **Rare Disease UK**: www.raredisease.org.uk — For genetic causes

### Questions to Ask Your Doctor

- What is causing my low calcium?
- Will it get better or do I need lifelong treatment?
- What medications do I need and for how long?
- How often do I need blood tests?
- Are there any foods or other medications I should avoid?
- What symptoms should I watch out for?
- Who should I contact if I have problems?

---

## References

### Primary Guidelines and Systematic Reviews

1. Cooper MS, Gittoes NJ. Diagnosis and management of hypocalcaemia. BMJ. 2008;336(7656):1298-1302. doi:10.1136/bmj.39582.589433.BE PMID: [18535071](https://pubmed.ncbi.nlm.nih.gov/18535071/)

2. Christakos S, Dhawan P, Verstuyf A, Verlinden L, Carmeliet G. Vitamin D: Metabolism, Molecular Mechanism of Action, and Pleiotropic Effects. Physiol Rev. 2016;96(1):365-408. doi:10.1152/physrev.00014.2015 PMID: [26681795](https://pubmed.ncbi.nlm.nih.gov/26681795/)

3. Tohme JF, Bilezikian JP. Hypocalcemic emergencies. Endocrinol Metab Clin North Am. 1993;22(2):363-375. PMID: [8325292](https://pubmed.ncbi.nlm.nih.gov/8325292/)

4. Shoback D. Clinical practice. Hypoparathyroidism. N Engl J Med. 2008;359(4):391-403. doi:10.1056/NEJMcp0803050 PMID: [18650515](https://pubmed.ncbi.nlm.nih.gov/18650515/)

5. Thakker RV. Genetic developments in hypoparathyroidism. Lancet. 2001;357(9269):974-976. doi:10.1016/S0140-6736(00)04268-8 PMID: [11293642](https://pubmed.ncbi.nlm.nih.gov/11293642/)

6. Fong J, Khan A. Hypocalcemia: updates in diagnosis and management for primary care. Can Fam Physician. 2012;58(2):158-162. PMID: [22439169](https://pubmed.ncbi.nlm.nih.gov/22439169/)

7. Brandi ML, Bilezikian JP, Shoback D, et al. Management of Hypoparathyroidism: Summary Statement and Guidelines. J Clin Endocrinol Metab. 2016;101(6):2273-2283. doi:10.1210/jc.2015-3907 PMID: [26943721](https://pubmed.ncbi.nlm.nih.gov/26943721/)

8. Rude RK, Singer FR, Gruber HE. Skeletal and hormonal effects of magnesium deficiency. J Am Coll Nutr. 2009;28(2):131-141. doi:10.1080/07315724.2009.10719764 PMID: [19828898](https://pubmed.ncbi.nlm.nih.gov/19828898/)

9. Urbano FL. Signs of hypocalcemia: Chvostek's and Trousseau's signs. Hosp Physician. 2000;36:43-45.

10. Asari R, Passler C, Kaczirek K, Scheuba C, Niederle B. Hypoparathyroidism after total thyroidectomy: a prospective study. Arch Surg. 2008;143(2):132-137. doi:10.1001/archsurg.2007.55 PMID: [18283138](https://pubmed.ncbi.nlm.nih.gov/18283138/)

11. Witteveen JE, van Thiel S, Romijn JA, Hamdy NA. Hungry bone syndrome: still a challenge in the post-operative management of primary hyperparathyroidism: a systematic review of the literature. Eur J Endocrinol. 2013;168(3):R45-R53. doi:10.1530/EJE-12-0528 PMID: [23152439](https://pubmed.ncbi.nlm.nih.gov/23152439/)

12. Bushinsky DA, Monk RD. Electrolyte quintet: Calcium. Lancet. 1998;352(9124):306-311. doi:10.1016/S0140-6736(97)12331-5 PMID: [9690425](https://pubmed.ncbi.nlm.nih.gov/9690425/)

13. Lier H, Krep H, Schroeder S, Stuber F. Preconditions of hemostasis in trauma: a review. The influence of acidosis, hypocalcemia, anemia, and hypothermia on functional hemostasis in trauma. J Trauma. 2008;65(4):951-960. doi:10.1097/TA.0b013e318187e15b PMID: [18849817](https://pubmed.ncbi.nlm.nih.gov/18849817/)

14. Kochhar A, Larian B, Azizzadeh B. Facial nerve and parotid gland anatomy. Otolaryngol Clin North Am. 2016;49(2):273-284. doi:10.1016/j.otc.2015.10.002 PMID: [26614826](https://pubmed.ncbi.nlm.nih.gov/26614826/)

15. Riggs JE. Neurologic manifestations of fluid and electrolyte disturbances. Neurol Clin. 2002;20(1):227-239. doi:10.1016/s0733-8619(03)00060-0 PMID: [11754308](https://pubmed.ncbi.nlm.nih.gov/11754308/)

16. Kidney Disease: Improving Global Outcomes (KDIGO) CKD-MBD Update Work Group. KDIGO 2017 Clinical Practice Guideline Update for the Diagnosis, Evaluation, Prevention, and Treatment of Chronic Kidney Disease-Mineral and Bone Disorder (CKD-MBD). Kidney Int Suppl. 2017;7(1):1-59. doi:10.1016/j.kisu.2017.04.001 PMID: [30675420](https://pubmed.ncbi.nlm.nih.gov/30675420/)

17. Holick MF. Vitamin D deficiency. N Engl J Med. 2007;357(3):266-281. doi:10.1056/NEJMra070553 PMID: [17634462](https://pubmed.ncbi.nlm.nih.gov/17634462/)

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- **Last Updated**: 2025-01-07