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.
Insulin levels may affect the disorder in some people because insulin increases the flow of potassium into cells. Weakness most commonly affects the muscles of the arms and legs. Heart arrhythmias may occur during attacks because of the drop in potassium levels. Although muscle strength is initially normal between attacks, repeated attacks may eventually cause progressive and persistent muscle weakness. Risks include a family history of periodic paralysis and hyperthyroidism.