Low potassium in the bloodstream (serum potassium levels are normal between attacks)
Weakness, especially in the muscles of the arms and legs and occasionally in the muscles of the eyes. The muscles involved in breathing and swallowing can very rarely be affected, and this can be fatal. Muscle strength is normal between attacks at first. However, repeated attacks may eventually cause worsening and persistent muscle weakness.
Between attacks, the examination is normal. Or, there may be signs of hyperthyroidism, such as an enlarged thyroid.
The following tests are used to diagnose hyperthyroidism:
The best treatment is to quickly reduce thyroid hormone levels. Potassium should also be given during the attack, usually by mouth. If weakness is severe, you may need to get potassium through a vein (intravenously). (You should only get intravenous potassium if your kidney function is normal and you are monitored in the hospital.)
Weakness that involves the muscles used for breathing or swallowing is an emergency. Patients must be taken to a hospital. Dangerous heart arrhythmias may also occur during attacks.
Your health care provider may recommend eating a diet that is low in carbohydrates and salt to prevent attacks. Medications called beta-blockers may reduce the number and severity of attacks while your hyperthyroidism is brought under control.
Acetazolamide is effective at preventing attacks in people with familial periodic paralysis. It is usually not effective for thyrotoxic periodic paralysis.
Chronic attacks over time can lead to muscle weakness. This weakness can continue even between attacks if the thyrotoxicosis is not treated.
Thyrotoxic periodic paralysis responds well to medical treatment. Treating hyperthyroidism will prevent attacks and may even reverse muscle weakness.
Difficulty breathing, speaking, or swallowing during attacks (rare)
Heart arrhythmias during attacks
Muscle weakness that gets worse over time
When to Contact a Medical Professional
Go to the emergency room or call the local emergency number (such as 911) if you have periods of muscle weakness. This is especially important if you have a family history of periodic paralysis or thyroid disorders.
Emergency symptoms include:
Difficulty breathing, speaking, or swallowing
Falls due to muscle weakness
Genetic counseling may be advised. Treating the thyroid disorder prevents attacks of weakness.
Mandel SJ, Larsen PR, Davies F. Thyrotoxicosis. In: Melmed S. Polonsky KS, Larsen PR, Kronenberg HM, eds. Williams Textbook of Endocrinology. 12th ed. Philadelphia, PA: Elsevier Saunders; 2011:chap 12.
Klubo-Gwiezdzinka J, Wartofsky L. Thyroid emergencies. Med Clin N Am. 2012;96:385-403.
Brent Wisse, MD, Associate Professor of Medicine, Division of Metabolism, Endocrinology & Nutrition, University of Washington School of Medicine, Seattle, WA. Also reviewed by David Zieve, MD, MHA, Isla Ogilvie, PhD, and the A.D.A.M. Editorial team.