Thyrotoxic periodic paralysis is a complication of hyperthyroidism where the increase in thyroid hormones depletes potassium and causes muscle weakness. In severe cases, it can weaken the muscles that control breathing leading to respiratory arrest, or can cause a fatal arrythmia.
Although individuals suffering an attack may also exhibit classic signs of hyperthyroidism, about half of cases don't show any sign of the illness.
Not all sufferers of hyperthyroidism have the complication, and it appears to have a genetic component. It is more common with Grave's disease than with other forms of hyperthyroidism. It is far more common in male patients of East Asian ancestry.
Attacks generally start with muscle pain, cramping and stiffness, followed by rapidly progressing weakness or paralysis. Attacks are most common after waking or just before going to sleep. Exertion, alcohol, or food high in carbohydrates or salt can also trigger an attack. Without treatment, the condition will usually resolve itself, but this may take several days so it should be treated as a medical emergency.
Low potassium levels are typical of patient suffering from an attack, and many have elevated creatine kinase levels as well. Immediate treatment is potassium, but as reocurrence is likely, ongoing treatment should focus on the hyperthyroidism as treating that will usually stop the attacks.