Suspected hyperthyroidism‐associated myoclonus in a cat

A 10‐year‐old, male, neutered domestic shorthair cat presented with a 2‐week history of progressively worsening myoclonic jerks predominantly involving the head. Jerks were continuous and marked at rest, but became less intense during activity. Bloodwork, including haematology, biochemistry, bile acid stimulation test, ionised calcium and serology antibody titres for infectious diseases, was normal, except for high serum total thyroxine (TT4). Magnetic resonance imaging of the brain was normal. Thoracic radiographs revealed mild cardiomegaly. The cat was initially treated with thiamazole, showing gradual resolution of jerks, which returned when it was discontinued. Following radioactive iodine treatment, the jerks resolved completely within 3 months, with normalisation of serum TT4 levels. Electroencephalography with time‐locked video captured the myoclonus; however, no epileptiform activity was detected during the jerks. To our knowledge, this is the first report of hyperthyroidism‐associated myoclonus in a cat.