A Case of Thyrotoxic Periodic Paralysis

Introduction: Thyrotoxic periodic paralysis (TPP) is an uncommon disorder characterized by simultaneous thyrotoxicosis, hypokalemia, and paralysis. It is a rare complication of hyperthyroidism with a prevalence of 1 in 100,000. It has a higher prevalence in young Asian males, with much fewer cases reported in Hispanic patients. The majority of cases are seen in hyperthyroidism due to Grave’s disease, however other causes of thyrotoxicosis have been associated with TPP. Hypokalemia occurs when thyroid hormone stimulates the sodium-potassium-ATP pump by binding to the thyroid response elements upstream of the genes for this pump, increasing its activity and thereby causing a transcellular shift of potassium into the intracellular fluid. Attacks usually begin with proximal muscle weakness of the lower extremities and may progress to tetraplegia, with the degree of muscle weakness corresponding to serum potassium levels. Interestingly, no correlations with serum T3 or T4 levels have been found. Clinical Case: A 38-year-old Hispanic man with a history of GERD presented to the emergency department with an inability to move his extremities. Initial labs were significant for profound hypokalemia to 2.0 mEq/L (3.5-5) and hyperthyroidism, with TSH<0.05 uU/mL (0.3-4.2) and free T4 2.4 ng/dL (0.6-1.5). Additional workup revealed a positive thyroid stimulating immunoglobulin and a positive thyrotropin binding inhibitor, and thyroid ultrasound demonstrated a hyperemic thyroid with numerous subcentimeter hypoechoic nodules, all of which was suggestive of Grave’s disease. His symptoms resolved with potassium repletion, suggesting thyrotoxic periodic paralysis secondary to hyperthyroidism. Upon further evaluation, the patient endorsed several months of hyperthyroid symptoms such as tremors, heat intolerance, and weight loss. He also endorsed alcohol use, a high carbohydrate diet, and recent life stressors, a combination of which likely precipitated his thyrotoxic periodic paralysis. He was discharged on methimazole 10 mg daily and propranolol 40 mg BID, with a decrease in his free T4 and improvement in his symptoms. He has been educated to avoid alcohol consumption and high-carbohydrate meals to avoid precipitating another episode of periodic paralysis, and once his hyperthyroidism is under better control he plans to undergo radioactive iodine ablation (RAI) for definitive management. Conclusions: Thyrotoxic periodic paralysis is a rare but dangerous complication of hyperthyroidism, and should be considered in the differential diagnosis when young individuals present with otherwise unexplained hypokalemia and paralysis. Acute management is potassium repletion, long term management involves treatment of thyrotoxicosis and avoiding precipitating factors, and definitive therapy is RAI or thyroidectomy.