Signs and Symptoms of Thyrotoxic Periodic Paralysis
Thyrotoxic periodic paralysis (TPP) is characterized by sudden episodes of muscle weakness and hypokalemia in the setting of thyrotoxicosis, most commonly affecting young Asian and Hispanic males with subtle signs of hyperthyroidism.
Clinical Presentation
Primary Symptoms
- Sudden onset of muscle weakness, typically more severe in the lower extremities than upper extremities 1, 2
- Proximal muscle weakness more pronounced than distal muscle weakness 1
- Episodes often occur at night or after prolonged fasting 3
- Attacks are transient and self-limited 4
Associated Symptoms
- Patients may have minimal or subtle symptoms of hyperthyroidism despite abnormal thyroid function tests 1, 5
- When present, thyrotoxic symptoms may include:
Cardiovascular Findings
- Systolic hypertension 2
- Tachycardia 2, 4
- QTc prolongation (present in up to 89% of TPP cases) 5
- First-degree atrioventricular block may be seen on ECG 2
- Ventricular tachycardia (rare but serious complication) 3
Laboratory Findings
- Hypokalemia (typically < 3.5 mmol/L) is the hallmark finding, though cases with normal potassium have been reported 1, 2
- Low urinary potassium excretion 2
- Hypophosphatemia with hypophosphaturia 2
- Hypercalciuria 2
- Thyroid function tests showing:
Rare but Serious Complications
- Respiratory muscle involvement leading to acute hypercapnic respiratory failure 3
- Cardiac arrhythmias including ventricular tachycardia 3, 5
- Total quadriplegia (in severe cases) 4
Diagnostic Clues
- Presentation in adult males with no family history of periodic paralysis 2
- Episodes that resolve spontaneously or with potassium supplementation 4
- Attacks that occur after high carbohydrate meals, stress, or exercise 2
- Lower Burch-Wartofsky score compared to patients with complicated thyrotoxicosis (19 vs. 35) 5
- Electromyography showing low-amplitude electrical compound muscle action potential 2
Differential Diagnosis
- Familial hypokalemic periodic paralysis 4
- Hyperthyroidism without periodic paralysis 6
- Metabolic disorders affecting calcium-phosphate metabolism 6
- Glucose metabolism disorders 6
- Other causes of acute muscle weakness 6
Clinical Pearls
- TPP should be considered in any young Asian or Hispanic male presenting with acute muscle weakness, even if signs of hyperthyroidism are subtle or absent 1, 5
- The condition is completely treatable and curable with proper management of the underlying hyperthyroidism 2
- Immediate therapy with potassium chloride supplementation can rapidly reverse muscle weakness, but carries risk of rebound hyperkalemia 2
- Non-selective beta-blockers may provide an alternative treatment option 6, 2