This paper discusses the design and development of our decision support system for thyroid function diagnostics, named Thyroid. The objective of our work has been to develop a decision support system for practical use. The Thyroid system has been designed to utilize the existing laboratory test results of the patient. The task of the system is to produce interpretative reports for the clinicians. The system is integrated with the laboratory information system of University Central Hospital of Turku. It has been under clinical testing for more than 2 years with over 1600 actual cases. The system has produced promising results and it has also shown the limitations of the knowledge-based systems in this infrastructure environment.