Familial periodic paralysis is a disease characterized by sudden attacks of weakness and paralysis. Weakness is recurrent, affecting mainly the limbs, and is often brought on by exercising or eating too many or too few carbohydrates. There are 4 forms of Familial periodic paralysis: hypokalemic, hyperkalemic, thyrotoxic, and Andersen-Tawil syndrome. In the hypokalemic form, the paralysis is caused by low levels of potassium. In the hyperkalemic form, the paralysis is caused by high levels of potassium in the blood. In the thyrotoxic form, the paralysis is caused by low levels of potassium in the blood and an overactive thyroid gland (hyperthyroidism). In Andersen-Tawil syndrome, potassium levels can be high, low, or normal. Genetic changes in the CACNA1S and SCN4A genes cause hypokalemic periodic paralysis. The hyperkalemic form is due to genetic changes in SCN4A gene. The underlying cause of the thyrotoxic form is unknown. Genetic changes in the KCNJ2 gene cause Andersen-Tawil syndrome.