Thyrotoxic periodic paralysis is a condition in which there are episodes of severe muscle weakness. It occurs in people who have high levels of thyroid hormone in their blood (hyperthyroidism, thyrotoxicosis).
This is a rare condition that occurs only in people with high thyroid hormone levels (thyrotoxicosis). Men of Asian or Hispanic descent are affected more often. Most people who develop high thyroid hormone levels are not at risk of periodic paralysis.
There is a similar disorder, called hypokalemic, or familial, periodic paralysis. It is an inherited condition and not related to high thyroid levels, but has the same symptoms.
Risk factors include a family history of periodic paralysis and hyperthyroidism.
Symptoms involve attacks of muscle weakness or paralysis. The attacks alternate with periods of normal muscle function. Attacks often begin after symptoms of hyperthyroidism have developed. Hyperthyroid symptoms may be subtle.
The frequency of attacks varies from daily to yearly. Episodes of muscle weakness may last for a few hours or several days.
The weakness or paralysis:
- Comes and goes
- Can last from a few hours up to several days (rare)
- Is more common in the legs than the arms
- Is most common in the shoulders and hips
- Is triggered by heavy, high-carbohydrate, high-salt meals
- Is triggered during rest after exercise
Other rare symptoms may include any of the following:
- Difficulty breathing
- Speech difficulty
- Swallowing difficulty
- Vision changes
People are alert during attacks and can answer questions. Normal strength returns between attacks. Muscle weakness may develop over time with repeated attacks.
Symptoms of hyperthyroidism include:
Exams and Tests
The health care provider may suspect thyrotoxic periodic paralysis based on:
- Abnormal thyroid hormone levels
- A family history of the disorder
- Low potassium level during attacks
- Symptoms that come and go in episodes
Diagnosis involves ruling out disorders associated with low potassium.
The provider may try to trigger an attack by giving you insulin and sugar (glucose, which reduces potassium level) or thyroid hormone.
The following signs may be seen during the attack:
- Decreased or no reflexes
- Heart arrhythmias
- Low potassium in the bloodstream (potassium levels are normal between attacks)
Between attacks, the examination is normal. Or, there may be signs of hyperthyroidism, such as an enlarged thyroid changes in the eyes, tremor, hair and nail changes.
The following tests are used to diagnose hyperthyroidism:
- High thyroid hormone levels (T3 or T4)
- Low serum TSH (thyroid stimulating hormone) levels
- Thyroid uptake and scan
Other test results:
- Abnormal electrocardiogram (ECG) during attacks
- Abnormal electromyogram (EMG) during attacks
- Low serum potassium during attacks, but normal between attacks
A muscle biopsy may sometimes be taken.
Potassium should also be given during the attack, most often by mouth. If weakness is severe, you may need to get potassium through a vein (IV). Note: You should only get IV 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. People must be taken to a hospital. Serious irregularity of heartbeat may also occur during attacks.
Your provider may recommend a diet that is low in carbohydrates and salt to prevent attacks. Medicines 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.
If an attack isn't treated and the breathing muscles are affected, death can occur.
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 treatment. Treating hyperthyroidism will prevent attacks. It may even reverse muscle weakness.
Untreated thyrotoxic periodic paralysis can lead to:
- 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
Call the local emergency number (such as 911) or go to the emergency room 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.
Periodic paralysis - thyrotoxic; Hyperthyroidism - periodic paralysis
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Kerchner GA, Ptacek LJ. Channelopathies: episodic and electrical disorders of the nervous system. In: Daroff RB, Jankovic J, Mazziotta JC, Pomeroy SL, eds. Bradley's Neurology in Clinical Practice. 7th ed. Philadelphia, PA: Elsevier; 2016:chap 99.
Selcen D. Muscle diseases. In: Goldman L, Schafer AI, eds. Goldman-Cecil Medicine. 26th ed. Philadelphia, PA: Elsevier; 2020:chap 393.
Review Date 1/26/2020
Updated by: Brent Wisse, MD, board certified in Metabolism/Endocrinology, Seattle, WA. Also reviewed by David Zieve, MD, MHA, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.