Methimazole is a thionamide drug that inhibits the synthesis of thyroid hormones by blocking the oxidation of iodine in the thyroid gland. We report a case of methimazole-induced recurrent pleural effusion. A 67-year-old female with recently diagnosed Graves' disease on methimazole 20mg daily was admitted with dyspnea and new onset atrial fibrillation with rapid ventricular rate. Chest X-ray revealed a unilateral right pleural effusion, which was consistent with a transudate on thoracocentesis. She was managed as a case of congestive heart failure and methimazole dose was increased to 30 mg daily. She was readmitted twice with recurrent right pleural effusion. The fluid revealed an exudative process on repeat thoracocentesis. CT scan of the chest with contrast showed mediastinal lymphadenopathy and a diffuse ground glass process involving the right lower lobe suggestive of pneumonitis. Bronchoalveolar lavage showed neutrophil predominant fluid, and cytology and adenosine deaminase were negative. Patient also had an endobronchial ultrasound guided biopsy of the lymph nodes (EBUS). She was treated empirically with steroids 40 mg for 10 days and the methimazole was also discontinued. The antinuclear antibodies (ANA) came back positive with a speckled pattern; antineutrophil cytoplasmic antibody (c-ANCA) and antimyeloperoxidase were also positive. The effusion resolved but recurred on rechallenge with methimazole. She was referred for urgent thyroidectomy. The patient's repeat chest X-ray showed complete resolution of the pleural effusion after stopping the methimazole. Few weeks later, repeat ANCA and antimyeloperoxidase antibody were both negative. Our case report highlights the importance of the recognition of a rare side effect of methimazole. Timely diagnosis would ensure that appropriate treatment is given.