A woman with Hashimoto thyroiditis, treated with levothyroxine for 4 years, complained of chest pain that radiated to the neck and had progressively worsened for 2 months. CT of the chest showed circumferential thickening of the ascending aorta and aortic arch, compatible with Takayasu arteritis. She underwent F-18 FDG PET imaging, which revealed an abnormal thyroid gland and a ring-like appearance of the mediastinum.