In other cases, the disorder appears sporadically in a family group. The disorder is most common in men, particularly Latin American and Asian men. The disorder involves attacks of muscle weakness or paralysis alternating with periods of normal muscle function.
Attacks usually begin after symptoms of hyperthyroidism have developed.
The frequency of attacks varies from daily to yearly. Episodes of muscle weakness may last for a few hours or may persist for several days. Serum potassium levels are normal between attacks. There is no decrease in total body potassium, however. Potassium flows from the bloodstream into muscle cells.