Thyrotoxic periodic paralysis (TPP) is a rare condition characterized by muscle paralysis, thyrotoxicosis, and hypokalemia. It presents with paralysis of both proximal and distal musculature in upper and lower limbs and may affect respiratory musculature and the cardiac conduction system. Early diagnosis is essential, as the condition is potentially reversible by oral or intravenous potassium treatment, leading to rapid resolution without lasting weakness. Overlooking the diagnosis may result in respiratory failure and cardiac arrhythmias including QT prolongation, Torsades de points, and ventricular arrhythmias.
A 19-year-old Caucasian man was admitted acutely with paralysis in upper and lower limbs and tachycardia. Over several months, he had experienced anxiousness, sweating more than usual, had daily palpitations, shortness of breath on exertion, and loose stools, and had lost 21 kg over the last year. Initial blood gas showed very low potassium of 1.4 mM, and blood tests showed decreased Thyroid-stimulating hormone (TSH) < 0.01 × 10 IU/L, elevated free thyroxine (fT4) of 63.5 pM (reference interval (RI): 12.0-22.0 pM), and elevated total triiodothyronine (T3) of 8.2 nM (RI: 1.0-2.6 nM). He was diagnosed with TPP and treated with liquid oral potassium chloride (30 mmol every 30 minutes) and propylthiouracil (initial dose of 400 mg followed by 200 mg three times daily). TSH-receptor antibodies (TRAB) and thyroid-peroxidase antibodies (TPO-ab) were highly elevated. Thyroid ultrasound showed a normal-sized gland and color Doppler sonography showed increased vascularity throughout the gland, compatible with Graves' disease. He was discharged on day 4 with a normal potassium level and followed in the outpatient clinic where he received standard care for Graves' disease. Genetic testing using whole-genome sequencing found no genetic variants in genes previously associated with TPP.
TPP is very rare in Caucasians but more often affects young men in East Asian populations. The case presents a Caucasian man with TPP where genetic testing of CACNA1S, KCNJ18, SCN4A, KCNJ2, KCNE3, and ABCC8 shows no pathogenic variants in genes previously associated with TPP.

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