Thyrotoxic periodic paralysis is a rare condition that occurs only in people with thyrotoxicosis (high thyroid hormone levels). It is seen most commonly in Asian men. There is a similar disorder, hypokalemic periodic paralysis (familial periodic paralysis), that is an inherited condition and is not associated with high thyroid levels.

Thyrotoxic periodic paralysis 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.

During an attack, there is a low level of potassium in the bloodstream (serum). Serum potassium levels are normal between attacks. There is no decrease in total body potassium, however.

Normally, potassium flows from the bloodstream into muscle cells.When levels of potassium are low (hypokalemia), there may not be enough for proper muscle function. Insulin levels may affect the disorder because insulin increases the flow of potassium into cells.

Weakness most commonly affects the muscles of the arms and legs. It may occasionally affect the muscles of the eyes. The muscles involved in breathing and swallowing can sometimes be affected and this can be fatal. Heart arrhythmias can also 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.

Risk factors include a family history of periodic paralysis and hyperthyroidism. Attacks may be triggered by eating high-carbohydrate or high-salt meals. Rest after vigorous exercise can also trigger an attack.