Thyroid Disorders USMLE Step 1 Practice Question
A 62-year-old man with newly diagnosed hypothyroidism started levothyroxine 50 mcg daily. Six weeks later, he presents with palpitations and irregular heartbeat. Vital signs show HR 118 bpm, BP 148/92 mmHg, RR 18, temperature 37.2°C, SpO2 98% on room air. ECG confirms atrial fibrillation. Labs reveal TSH 0.05 mIU/L (normal 0.4–4.0) and free T4 18 pg/dL (normal 7–20). Thyroid ultrasound shows no nodules. Which mechanism best explains his arrhythmia?
Answer choices
- ADirect cardiotoxic effect of levothyroxine metabolites
- BLevothyroxine dose is too high, causing iatrogenic thyrotoxicosisCorrect answer
- CReactivation of Graves disease in response to levothyroxine
- DAcute myocarditis triggered by levothyroxine
- EUnderlying cardiac arrhythmia unrelated to thyroid therapy
- FRapid conversion of T4 to T3 in cardiac tissue causing local thyroid hormone excess
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