A one-year-old Persian male cat was presented at the Small Animal Clinic, Assiut Veterinary Teaching Hospital for a general health check and advice on poor weight gain/stunted growth. Clinical examination revealed enlarged thyroid gland and un-descended testicles. Thyroxine (T4) level was above normal (90.09; reference range 40-70 nmol/l); urea level was markedly increased (39.98; reference range 3-15 mmol/l) and creatinine concentration was slightly below normal (143.2; reference range 150-180 umol/l) suggesting renal involvement. A normal total leukocytic count, neutrophilia with degenerative left shift was also detected. The owner declined medical treatment and euthanasia and the cat died one week from presentation at the clinic. Medical therapeutics (e.g. Carbimazole) and radioactive isotopes are two of the treatment options recommended for similar cases. Surgical excision is another option although not without the risk of resultant hypothyroidism. This is, to the author's knowledge, the first case report of juvenile/congenital hyperthyroidism in a Persian cat in Egypt.