Weight loss, tremor, and atrial fibrillation recurrence in a 68-year-old man on an antiarrhythmic agent

Admission note: A 68-year-old man was admitted with a recurrence of atrial fibrillation. He has a history of paroxysmal atrial fibrillation and hypertension. Eight months ago, he was started on amiodarone 200 mg daily following a maintenance protocol, in combination with digoxin 0.25 mg daily, warfarin 5 mg daily (dose adjusted to INR 2 to 3), and lisinopril 10 mg daily. He was stable at home until approximately two months ago, when he developed a fine tremor of the hands, unintentional weight loss of 6 kg, and episodes of palpitations. He also reported feeling anxious. On examination, he was in atrial fibrillation with a ventricular rate of 110 bpm, blood pressure 140/80 mmHg. There was a fine tremor in both hands, and his skin was warm and moist. No goiter or exophthalmos was noted. The rest of the examination was unremarkable.

Laboratory investigations: TSH less than 0.01 mIU/L, free T4 32 pmol/L, free T3 9 pmol/L. Thyroid peroxidase and TSH receptor antibodies were negative. A thyroid ultrasound showed a diffuse enlargement with increased vascularity, no discrete nodules. His digoxin level was 1.4 ng/mL (therapeutic range 0.8 to 2.0 ng/mL), and his INR was 2.8. A 12-lead ECG confirmed atrial fibrillation.

The patient's other medications (digoxin, warfarin, lisinopril) are known to have a low potential for causing thyroid dysfunction. A non-drug cause, such as Graves' disease, was considered, but the negative TSH receptor antibodies and the lack of eye signs made this less likely. After discussion with the endocrinology team, his amiodarone was discontinued, and he was started on propranolol 40 mg twice daily for rate control. Over the following weeks, his symptoms improved. Three months later, his thyroid function tests had normalised, and he remained in sinus rhythm.

Note: No rechallenge was performed.