Iatrogenic Primary Hypothyroidism Associated With Sulfamethoxazole-Trimethoprim Treatment of Nocardiosis in a Cat

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 (^99mTc-pertechnetate) uptake. Upon discontinuation of trimethoprim-sulfa, serum concentrations of T4, fT4, and TSH returned to normal, confirming the diagnosis of iatrogenic, drug-induced thyroid dyshormonogenesis leading to hypothyroidism. A skin lesion was surgically removed, and microscopy disclosed branched filaments along with characteristic Gram-positive coccobacilli and Splendore-Hoeppli phenomenon. Using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS), the clinical isolate was identified as Nocardia puris.