THE CASE
A 33-year-old Hispanic man with no significant past medical history presented to the emergency department with generalized flaccid paralysis in both arms and legs. Two days before, he had been working on a construction site in hot weather. The following day, he woke up with very little energy or strength to perform his daily activities, and he had pain in the inguinal area and both calves. He denied taking any medications or supplements.
The patient had complete muscle weakness and was unable to move his arms and legs. He reported dysphagia and an unintentional weight loss of 30 lb during the previous month.
On physical examination, the patient’s vital signs were within the normal range, and mild thyromegaly without nodules was present. Neurologic examination revealed decreased deep tendon reflexes with intact sensation. Muscle strength in his arms and legs was 0/5.
Initial laboratory test results included a potassium level of 2.2 mEq/L (normal range, 3.5–5 mEq/L) and normal acid-basic status that was confirmed by an arterial blood gas measurement. Serum magnesium was 1.6 mg/dL (normal range, 1.6–2.5 mg/dL); phosphorus, 1.9 mg/dL (normal range, 2.7–4.5 mg/dL); and random urinary potassium, 16 mEq/L (normal range, 25–125 mEq/L). An initial chest x-ray was normal, and an electrocardiogram showed a prolonged QT interval, flattening of the T wave, and a prominent U wave consistent with hypokalemia.
THE DIAGNOSIS
The initial clinical diagnosis was hypokalemic paralysis. The patient was treated with intravenous (IV) potassium chloride 40 mEq; however, his potassium level decreased further, to 1.8 mEq/L. Potassium chloride administration was continued and potassium levels were monitored. Normal saline 1 L was also administered, and other electrolyte abnormalities were corrected.
Evaluation of the patient’s hypokalemia revealed the following: thyroid-stimulating hormone (TSH) level, < 0.01 microIU/mL (normal range, 0.27–4.2 microIU/mL); free T4 (thyroxine) level, 4.47 ng/dL (normal range, 0.08–1.70 ng/dL); total T3 (triiodothyronine) level, 17.5 ng/dL (normal range, 2.6–4.4 ng/dL).
The patient was diagnosed with hypokalemic periodic paralysis (HPP) secondary to thyrotoxicosis, also known as thyrotoxicosis periodic paralysis (TPP). His hyperthyroidism was treated with oral atenolol 25 mg/d and oral methimazole 10 mg tid.
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