The complete hospital information system supports the electronic patient record, which, with the history registration, its laboratory and depiction examinations through the use of expert systems, leads to the immediate and effective diagnosis and treatment of the illness. In this present study with the use of a data flow diagram, which consists of a small indication of the way an expert system based on Artificial Intelligence can be made applicable, the steps which lead to the diagnosis of thyroid cancer will be mentioned, when the patient is admitted to the outpatient Endocrinology department. With the data flow diagram, the users can visualize the way in which the system will operate and what it can achieve. Also it will present the course which the medical professional follows in order to reach the diagnosis of thyroid cancer with the slightest error percentage, using the medical information in the extensive hospital information system.