Q1 - Post-Thyroidectomy Hypoparathyroidism & Hypocalcemia

Clinical Diagnosis: Hypoparathyroidism leading to Hypocalcemia (Tetany)

1. What is the cause of this condition? [2 marks]

• Calcium reabsorption in the renal distal tubule falls
• Bone resorption by osteoclasts decreases
• Renal activation of Vitamin D (1,25-diOH-D3/calcitriol) is impaired, reducing intestinal calcium absorption
• Serum calcium drops acutely - hypocalcemia results

2. What are the hormones regulating serum calcium? Discuss their MOA. [3 marks]

A. Parathyroid Hormone (PTH) - HYPERCALCEMIC hormone

B. Calcitriol (1,25-dihydroxyvitamin D3) - HYPERCALCEMIC hormone

• Acts like a steroid hormone - enters cells, binds cytosolic receptor, the complex moves to the nucleus and acts on DNA response elements
• Intestine: Increases expression of calbindin (calcium-binding protein) → enhances intestinal Ca²⁺ absorption (major effect)
• Bone: Stimulates bone resorption (mobilizes Ca²⁺ reserve), synergistic with PTH
• Kidney: Increases Ca²⁺ and phosphate reabsorption

C. Calcitonin - HYPOCALCEMIC hormone

• Bone: Inhibits osteoclast activity → decreases bone resorption → lowers serum Ca²⁺
• Kidney: Inhibits tubular reabsorption of Ca²⁺ → increases urinary calcium excretion
• Intestine: Reduces calcium absorption
• Net effect: Opposes PTH - lowers serum calcium

Note: In this patient, the thyroid was removed for malignancy, so calcitonin from C-cells is also lost - but since calcitonin is hypocalcemic, its loss would actually tend to raise calcium. The dominant problem is PTH loss.

Response to low serum calcium: PTH rises → stimulates calcitriol synthesis → calcium mobilization from bone, renal reabsorption, and intestinal absorption all increase. - Lippincott's Biochemistry, 8th ed.

3. What is the cause for symptoms seen in this case? [4 marks]

• Cramps in hand muscles
• Continuous twitching (fasciculations)
• Laryngeal spasm

1. Less Ca²⁺ is available to plug Na⁺ channels
2. Na⁺ channels become hyperpermeable
3. The threshold for nerve depolarization drops
4. Spontaneous repetitive firing of motor and sensory neurons occurs without any stimulus
5. This drives sustained, uncontrolled muscle contraction = tetany

• Chvostek's sign: Tapping the facial nerve (just anterior to the ear) causes twitching of facial muscles
• Trousseau's sign: Inflating a BP cuff above systolic pressure for 3 minutes causes carpal spasm (main d'accoucheur / "obstetrician's hand" posture)

4. Comment on the values obtained during blood and urine tests. [4 marks]

1. Low plasma Ca²⁺ + High plasma PO₄³⁻: PTH normally promotes urinary phosphate excretion (phosphaturia) and calcium reabsorption. Without PTH, phosphate is retained in blood (hyperphosphatemia) and calcium is not reabsorbed (hypocalcemia). As Comprehensive Clinical Nephrology states: "Loss of PTH (hypoparathyroidism) leads to hypocalcemia and hyperphosphatemia."
2. Low urine calcium: Even though serum calcium is low, the kidney still excretes some calcium. However, in hypoparathyroidism, absence of PTH means there is no signal to reabsorb calcium in the distal tubule - yet the filtered load of calcium is also very low (because serum Ca²⁺ is low). The net result is reduced urinary calcium. Additionally, there is no calcitriol-driven increased gut absorption to replenish the pool.
3. Low urine phosphorus: Without PTH, the proximal tubule reabsorbs phosphate avidly (phosphate reabsorption is the default state; PTH normally inhibits it). So despite elevated serum phosphorus, phosphaturia is low because PTH is absent and renal tubular phosphate reabsorption is high.

• Plasma: ↓ Ca²⁺ + ↑ PO₄³⁻ = Hypoparathyroidism
• Urine: ↓ Ca²⁺ + ↓ PO₄³⁻ = consistent with absent PTH

5. What are the tests done to detect this condition in its latent phase? [2 marks]

A. Bedside Clinical Tests

1. Chvostek's Sign
   • Tap the facial nerve just anterior to the ear lobe (over the parotid)
   • Positive: ipsilateral twitching of the corner of the mouth, nose, or eye
   • Indicates subclinical neuromuscular hyperexcitability
2. Trousseau's Sign
   • Inflate a sphygmomanometer above systolic BP on the arm for 3 minutes
   • Positive: carpal spasm = flexion of wrist and MCP joints, extension of IP joints, adduction of thumb ("obstetrician's hand")
   • More specific than Chvostek's for latent tetany
3. Peroneal Sign (Lust's Sign)
   • Tap over the peroneal nerve → dorsiflexion and eversion of foot

B. Laboratory Tests

1. Serum calcium (total and ionized): Serum Ca²⁺ below 8.8 mg/dL confirms hypocalcemia; ionized Ca²⁺ < 4.6 mg/dL is diagnostic
2. Serum PTH (intact PTH assay): Low or undetectable PTH in the setting of low calcium confirms hypoparathyroidism
3. Sulkowitch Test (Urine calcium): A simple bedside test - adding Sulkowitch reagent (oxalic acid) to urine generates a precipitate proportional to calcium content; absent/fine precipitate = low urine calcium = hypocalcemia
4. ECG: Prolonged QT interval (due to prolonged phase 2 of cardiac action potential from hypocalcemia) is an objective indicator
5. Serum phosphorus: Elevated in hypoparathyroidism

• Robbins & Cotran Pathologic Basis of Disease - Hypoparathyroidism, Clinical Features
• Barash's Clinical Anesthesiology, 9e - Calcium Physiology
• Lippincott's Biochemistry, 8th ed - Vitamin D and calcitriol regulation
• Comprehensive Clinical Nephrology, 7th ed - Hypoparathyroidism
• Sabiston Textbook of Surgery - Hypocalcemia
• Brenner & Rector's The Kidney - PTH-Vitamin D Endocrine System