A 9-year-old mixed breed cat with a history of recurrent ulcerated skin lesions was diagnosed with nocardiosis. Three months after initiating potentiated sulfonamide treatment, the cat developed goitrous hypothyroidism, characterized by palpable enlargement of both thyroid lobes, low serum concentrations of total thyroxine (T4) and free thyroxine (fT4), and high serum thyroid-stimulating hormone (TSH) concentration. Thyroid scintigraphy identified symmetrical enlargement of both thyroid lobes, with increased radionuclide (