Expert or knowledge-based systems are the most common type of AIM (artificial intelligence in medicine) system in routine clinical use. They contain medical knowledge, usually about a very specifically defined task, and are able to reason with data from individual patients to come up with reasoned conclusions. Although there are many variations, the knowledge within an expert system is typically represented in the form of a set of rules. The thyroid gland is one of the most important organs in the body as thyroid hormones are responsible for controlling metabolism. As a result, thyroid function impacts on every essential organ in the body. When the thyroid produces too much hormone, the body uses energy faster than it should. This condition is called hyperthyroidism. When the thyroid does not produce enough hormone, the body uses energy slower than it should. This condition is called hypothyroidism. Thyroid disease can be difficult to diagnose because symptoms are easily confused with other conditions. When thyroid disease is caught early, treatment can control the disorder even before the onset of symptoms. This study aims at diagnosing thyroid diseases with a expert system that we called as a ESTDD (expert system for thyroid disease diagnosis). We found fuzzy rules by using neuro fuzzy method, which will be emplaced in ESTDD system. ESTDD could diagnose with 95.33% accuracy thyroid diseases. Beside it can be benefited from this system for training of students in medicine.