Perirenal Pseudocyst formation is a relatively uncommon disease in cats. This paper presents a rare case of pseudocyst formation in cat because of multiple interdependent systems pathology. A 17-years-old Siberian cat suffering from hyperthyroidism for four years was presented to the Veterinary Teaching Hospital of Istanbul University with a history of anorexia, respiratory distress, and abdominal enlargement due to fluid accumulation. Hypertrophic cardiomyopathy and unilateral perirenal pseudocyst were diagnosed by echocardiography and abdominal ultrasonography, respectively. Abdominal fluid drained percutaneously was aseptic exudate with no signs of any type of neoplasia. Abdominal effusion developed after one month, and right sided nephrectomy was performed. Interstitial nephritis was diagnosed by histopathological examination. It was concluded that multiple system involvement in the form of hypertrophic cardiomyopathy, hyperthyroidism, and interstitial nephritis coupled with increasing age contributed toward formation of this perirenal pseudocyst in the cat.