Definition
Thyrotoxic periodic paralysis is a syndrome characterized by intermittent episodes of muscle weakness that occurs in people with high levels of thyroid hormone (hyperthyroidism, thyrotoxicosis).
Alternative Names
Periodic paralysis - thyrotoxic
Causes, incidence, and risk factors
Thyrotoxic periodic paralysis is a rare condition that occurs only in people with thyrotoxicosis (high thyroid hormone levels). It is seen most commonly in Asian men. There is a similar disorder, hypokalemic periodic paralysis (familial periodic paralysis), that is an inherited condition and is not associated with high thyroid levels.
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 may persist for several days.
During an attack, there is a low level of potassium in the bloodstream (serum). Serum potassium levels are normal between attacks. There is no decrease in total body potassium, however.
Normally, potassium flows from the bloodstream into muscle cells.When levels of potassium are low (hypokalemia), there may not be enough for proper muscle function. Insulin levels may affect the disorder because insulin increases the flow of potassium into cells.
Weakness most commonly affects the muscles of the arms and legs. It may occasionally affect the muscles of the eyes. The muscles involved in breathing and swallowing can sometimes be affected and this can be fatal.
Heart arrhythmias can also occur during attacks because of the drop in potassium levels. Although muscle strength is initially normal between attacks, repeated attacks may eventually cause progressive and persistent muscle weakness.
Risk factors include a family history of periodic paralysis and hyperthyroidism. Attacks may be triggered by eating high-carbohydrate or high-salt meals. Rest after vigorous exercise can also trigger an attack.
Symptoms
- weakness/paralysis
- most common in shoulders and hips
- more common in legs than arms
- intermittent occurrences
- triggered by rest after exercise
- triggered by heavy, high-carbohydrate, high-salt meals
- lasting for up to several days
- spontaneous recovery of normal strength between attacks
- vision changes (rare)
- swallowing difficulty (rare)
- speech difficulty (rare)
- difficulty breathing (rare)
- alert during attacks
Symptoms of hyperthyroidism:
Signs and tests
The health care provider may suspect thyrotoxic periodic paralysis based on a family history of the disorder, the episodic nature of symptoms, low potassium levels during attacks, abnormal thyroid hormone levels, and elimination of other disorders associated with low potassium as the cause of symptoms.
Between attacks, examination is normal, or there may be signs of hyperthyroidism, such as an enlarged thyroid. During an attack, reflexes may be decreased or absent. Weakness is constant rather than spastic (coming in spasms). The weakness is greater in muscle groups near the body (such as shoulders and hips) than muscle groups farther away from the body (such as arms, hands, legs, or feet). The health care provider may attempt to trigger an attack by administering insulin and glucose (which reduces potassium levels) or thyroid hormone.
Hyperthyroidism is confirmed by abnormal results of:
Serum potassium is low during attacks but is normal between attacks, confirming the diagnosis. An ECG (electrocardiogram) may be abnormal during attacks. An EMG (electromyogram) is normal between attacks. During an attack EMG is abnormal, showing electrical silence. A muscle biopsy may occasionally show abnormalities.
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