If the tongue is dry and the dryness is not relieved by drinking water, you may be suffering from oral mucosal dryness syndrome. The main symptoms of this disease, also called burning mouth syndrome, are extreme dryness in the mouth, a hot sensation on the surface of the tongue, pain in the mouth after prolonged speech, and difficulty in swallowing dry food. The cause of the disease is a serious impairment of salivary gland function due to immune dysfunction, resulting in a decrease in saliva production, which leads to severe dry mouth. If the tongue is very dry and water does not work, it may also be a manifestation of diabetes. Patients with diabetes will have an increased urine volume, and the increased urine volume will reduce water in the body, which will lead to a long-term dry mouth. In addition, in patients with hyperthyroidism, the body is prone to sweating, making the body fluid less, which also leads to dry mouth.