Thyrotoxic periodic paralysis is a condition in which there are episodes of muscle weakness in people with high levels of thyroid hormone (hyperthyroidism, thyrotoxicosis).

Thyrotoxic periodic paralysis is a rare condition that occurs only in people with high thyroid hormone levels (thyrotoxicosis). It is seen most commonly in Asian men.

There is a similar disorder, hypokalemic periodic paralysis (familial periodic paralysis). This is an inherited condition and does not have high thyroid levels.

Risk factors include a family history of periodic paralysis and hyperthyroidism.

Thyrotoxic periodic paralysis involves attacks of muscle weakness or paralysis alternating with periods of normal muscle function. Attacks usually begin after symptoms of hyperthyroidism have developed.

The frequency of attacks varies from daily to yearly. Episodes of muscle weakness may last for a few hours or several days.

Symptoms include:

• Difficulty breathing (rare)
• Speech difficulty (rare)
• Swallowing difficulty (rare)
• Vision changes (rare)
• Weakness/paralysis
  • Comes and goes
  • Lasts for up to several days
  • More common in legs than arms
  • Most common in shoulders and hips
  • Triggered by heavy, high-carbohydrate, high-salt meals
  • Triggered by rest after exercise

People are alert during attacks. Normal strength returns between attacks.

Symptoms of hyperthyroidism:

• Excessive sweating (diaphoresis)
• Fast heart rate
• Fatigue
• Headache
• Heat intolerance
• Increased appetite
• Insomnia
• Sensation of feeling the heart beat (palpitations)
• Skin changes: moist, warm, thin, pale (occasionally)
• Tremors
• Weight loss

The health care provider may suspect thyrotoxic periodic paralysis based on:

• Abnormal thyroid hormone levels
• A family history of the disorder
• Low potassium levels during attacks
• Symptoms that come and go in episodes

Diagnosis involves ruling out disorders associated with low potassium.

The health care provider may attempt to trigger an attack by giving you insulin and glucose (which reduces potassium levels) or thyroid hormone.

During an attack, there may be the following signs:

• Decreased or no reflexes
• Heart arrhythmias
• 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 sometimes 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, examination is normal. Or, there may be signs of hyperthyroidism, such as an enlarged thyroid.

Hyperthyroidism is confirmed by abnormal results on the following tests:

• High thyroid hormone levels (T3 or T4)
• Low serum TSH (thyroid stimulating hormone) levels

Other test results:

• Abnormal ECG (electrocardiogram) during attacks
• Abnormal EMG (electromyogram) during attacks
• Low serum potassium during attacks, but normal between attacks

A muscle biopsy may occasionally show abnormalities.

The best treatment is to quickly reduce thyroid hormone levels. Potassium should also be given during the attack, preferably by mouth. If weakness is severe, intravenous potassium may be needed. (Note: intravenous potassium should be given only if kidney function is normal and the patient is 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 that you eat a diet low in carbohydrates and salt to prevent attacks. Medications called beta blockers may reduce the number and severity of attacks while hyperthyroidism is brought under control.

Acetazolamide is effective in preventing attacks in people with familial periodic paralysis. It is usually not effective with thyrotoxic periodic paralysis.

Chronic attacks will eventually lead to muscle weakness that persists even between attacks. Thyrotoxic periodic paralysis responds well to 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

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
• Fainting

Genetic counseling may be advised. Treating the thyroid disorder prevents attacks of weakness.