Orbital lymphoma simulating thyroid orbitopathy

An 82-year-old man with hypothyroidism had vertical diplopia and swelling around his left eye. Visual acuity was 20/20 OD and 20/50 OS. There was moderate blepharoptosis and edema of the left eyelids and superior scleral show of the right eye. The left eye showed 4 mm of proptosis, motility restriction, afferent pupillary defect, and normal optic disc. Orbital MRI revealed enlargement of the left superior and medial rectus muscles without tendinous involvement. These findings were initially suggestive of thyroid orbitopathy. Thyroid function tests were normal. Coronal MRI showed additional superior oblique enlargement and involvement of the levator superioris palpebrae muscle, which are both suggestive of a non-thyroid pathology. Muscle biopsy revealed large B-cell lymphoma. The patient was treated with chemotherapy, immunotherapy, and radiotherapy, with complete tumor control. Orbital lymphoma can simulate thyroid orbitopathy, even in patients with classic "thyroid-like" symptoms and imaging.