Clinical hypothyroidism in a cat associated with sulfonamide administration for the management of intracranial nocardiosis

A 2-year-old cat was referred for suspected generalised seizure activity and reclusive behaviour, with a history of non-resolving facial abscess. Magnetic resonance imaging (MRI) revealed a contrast enhancing lesion occupying the left calvarium and adjacent peripheral tissues. The intracranial lesion was causing significant mass effect, with oedema and transtentorial herniation. Nocardia nova was isolated from the lesion and identified by DNA sequencing. Treatment consisted of debridement via craniotomy and ventral bulla osteotomy, and combination antibiotic therapy with clarithromycin, amoxycillin and trimethoprim-sulfonamide (sulfadoxine parenterally, then sulfadiazine orally). After several weeks of antibiotic therapy, the cat developed weakness, bicavitary effusion, myxoedema, non-regenerative anaemia and azotaemia. Total thyroxine (TT4) was below the detectable limit and canine thyroid stimulating hormone (cTSH) assay was markedly elevated at 7.53 ng/mL (reference interval 0.15–0.3 ng/mL). Discontinuation of sulfonamides and administration of levothyroxine resulted in resolution of clinical signs. The cat was subsequently able to discontinue levothyroxine, with recovery of euthyroid state. To the authors' knowledge, this is the first report of clinical hypothyroidism in a cat treated with sulfonamide antibiotics and may influence antimicrobial selection and monitoring during therapy. This report also described the management of an atypical presentation of nocardiosis with intracranial extension.