Sinus node dysfunction is one of the most common arrhythmias in elderly patients; it is usually associated with intermittent and variable symptoms, thus making it difficult to diagnose. We present the case of an elderly female patient with a personal history of atrial fibrillation treated for the last three years with amiodarone; she was admitted to the Geriatric Clinic for non-specific symptoms with onset two months previously for which she had already sought care in different medical services. Clinical examination showed severe bradycardia; ECG and Holter ECG on admission confirmed severe bradycardia, with a heart rate between 29 and 50 beats/min (bpm). Given her long-term treatment with amiodarone we looked for and found hyperthyroidism; the endocrine examination led to the diagnosis of mixed type Amiodarone-induced thyrotoxicosis and initiation of corticosteroid and antithyroid treatment. The evolution of cardiac arrhythmia was monitored with the help of several Holter ECGs performed after amiodarone washout and return to the euthyroid state, which revealed a tachycardia-bradycardia syndrome initially masked by the side effects of the unsupervised therapy with amiodarone, and properly treated by the implantation of a pacemaker.