Images of Hyperthyroidism: Thyrotoxic Periodic Paralysis

A 17-year-old adolescent experienced an episode of generalized paralysis of acute onset after physical exertion and consumption of a high-carbohydrate meal. Despite a good appetite, the patient had lost between 2 and 3 lb during the past month. He complained of heat intolerance and palpitations and reported losing his temper easily. Tremor in both hands, lid lag, and slight exophthalmos were noted; pulse rate was 88 beats per minute. The thyroid gland was 2 1/2 times the normal size. Potassium level was 1.8 mmol/L (normal, 3.5 to 5 mmol/L); bicarbonate, 19 mmol/L (normal, 21 to 31 mmol/L); free thyroxine, 49.4 pmol/L (normal, 8 to 22 pmol/L); and thyroid-stimulating hormone, less than 0.05 mU/L (normal, 0.2 to 6 mU/L). Thyrotoxic periodic paralysis was diagnosed. Periodic paralysis is a rare myopathic disorder that is characterized by episodic attacks of symmetric flaccid paralysis that are typically precipitated by exposure to cold, high-carbohydrate meals, rest after exercise, alcohol, infection, trauma, menstruation, or emotional excitement. The serum potassium level may be normal, low, or high during paralytic attacks. Although periodic paralysis is usually familial, sporadic cases may be associated with hyperthyroidism. It has been suggested that excessive amounts of thyroid hormone increase the permeability of the muscle membranes to electrolytes, which results in an influx of potassium into the cells. This patient was given methimazole, 25 mg/d. His thyroid function test results and serum potassium level normalized; the periodic paralysis did not recur.