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At a glance

An acquired, sporadic disorder characterized by intermittent episodes of muscle weakness alternating with periods of normal muscular function. It occurs during hyperthyroidism and thyrotoxicosis. During attack, hypokalemia is present. Hyperinsulinemia is also observed in acute attack.

Synonym

Hashitoxic Periodic Paralysis.

Incidence

Most common in Chinese and Japanese. The incidence is estimated at 1.9 to 8.8% of hyperthyroid patients. The disorder is rare in non-Asian populations and only 5% of cases occur in women. A few cases have been reported in African Americans, and this disorder may be more frequent in black patients. The male-to-female ratio ranges from 17:1 to 70:1 despite the fact that hyperthyroidism is more common in females (female-to-male ratio of 9:1).

Genetic inheritance

Mutations in Kir2.6 channel leading to loss of function predispose patients to an acute attack and may have role in the pathogenesis.

Pathophysiology

Exact pathophysiology remains to be defined. The following factors appear to be important in the pathogenesis: (a) hyperthyroidism increases Na/K pump ATPase activity, (b) increased platelet Na/K pump ATPase activity has been demonstrated in patients with thyrotoxic periodic paralysis, as compared with thyrotoxic patients, (c) thyroxine sensitizes beta-adrenergic receptors to the effects of catecholamines (Na/K pump ATPase activity is increased), (d) insulin increases Na/K ATPase activity, which may explain the relationship of acute episodes of paralysis to carbohydrate ingestion. These factors result in an intracellular shift of potassium and reduction of the extracellular potassium, causing membrane hyperpolarization. Some of the changes are present in patients who do not develop thyrotoxic periodic paralysis; therefore, a role for Na/K ATPase-independent potassium influx has been suggested and the role of Ca2+ gated ion channels is under investigation. The sarcolemmal membrane is thought to be the site of the primary defect. Hypophosphatemia is occasionally found in conjunction with thyrotoxic periodic paralysis and may be synergistic in producing paralysis.

Diagnosis

Demonstration of clinical or subclinical hyperthyroidism associated with muscle weakness or paralysis and hypokalemia (usually <3.0 mmol/L and as low as 1.1 mmol/L).

Clinical aspects

Thyrotoxic Periodic Paralysis is uncommon in childhood, with approximately 80% of cases presenting after the age of 20 years. Symptoms include myalgia, fatigue, muscle weakness, and symptoms of hyperthyroidism, although these are often subtle. There may be a history of weakness related to exercise and carbohydrate or alcohol intake. Proximal muscles are more affected than distal muscles, and lower limbs more than upper limbs. Examination commonly reveals a goiter and flaccid paralysis in the affected muscles. Respiratory and bulbar muscles are rarely involved. Cardiac dysrhythmias (atrial fibrillation/flutter, supraventricular tachycardia ventricular extrasystoles) are common. Electrocardiographic changes reflect the hypokalemia (U waves, small T wave, ST depression, long QT interval). Sinus tachycardia which is not normally present in hypokalemia. Definitive ...

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