Familial hypokalemic periodic paralysis is a rare autosomal dominant neuromuscular disorder typically found in adolescent males (due to mutations of CACNA1s and SCN4A). It is characterized by episodes of painless, often progressive muscle weakness or paralysis precipitated by heavy exercise, fasting, or high-carbohydrate meals or upon awakening.

This paralysis, unlike hyperkalemic periodic paralysis, is associated with low potassium levels (< 2.4 mEq/L; < 2 suggests a secondary cause of hypokalemia). Established familial history is diagnostic.

Likely a molecular subtype of Graves disease, thyrotoxic periodic paralysis (TPP) is associated with hypokalemia as well as hyperthyroidism. Its clinical episodes are similar to those of familial hypokalemic periodic paralysis. Distinguishing between these 2 diagnoses is important for prognosis and treatment.