Objective. To describe a patient with unilateral exudative pleural effusion that developed after commencement of carbimazole. Methods. We describe the presentation and clinical journey of an elderly woman who presented to the chest physicians initially with pleural effusion but was followed up by the endocrinology team. Result. The patient was a 77-year-old Caucasian woman who presented with symptoms of breathlessness and a confirmed unilateral pleural effusion while being on treatment for thyrotoxicosis. Her symptoms needed recurrent hospital admission for investigations and drainage, but no potential cause was identified after extensive investigations. A drug-induced exudative effusion consequent to carbimazole intake was diagnosed as discontinuation of the drug lead to complete resolution of the effusion with no recurrence. Conclusion. Physicians and Endocrinologist must bear in mind that this potentially rare complication of carbimazole while treating patients of thyrotoxicosis as appearance of similar features in their patients while being on carbimazole should lead to the discontinuation of the drug, and alternative treatment strategy should be considered.