Hyperthyroidism in Disguise: A Case of New-Onset Atrial Fibrillation and Acute Reversible Paralysis.

Thyrotoxic periodic paralysis (TPP) is a rare and potentially life-threatening disorder characterized by transient muscle weakness due to hypokalemia in the setting of hyperthyroidism. We present a case of a 32-year-old male with no prior history of thyroid disease who developed acute, progressive paralysis of all 4 extremities. Laboratory findings revealed profound hypokalemia, hypophosphatemia, and elevated lactate. Electrocardiography showed new-onset atrial fibrillation with a rapid ventricular response. Thyroid function tests confirmed hyperthyroidism, and further evaluation led to the diagnosis of Graves disease. The patient was treated with intravenous potassium and phosphorus replacement, leading to full neurological recovery within 24 hours. He was subsequently started on carbimazole for long-term thyroid hormone suppression. The occurrence of atrial fibrillation in TPP is an uncommon yet significant complication. Additionally, elevated lactate is an extremely rare finding.