Constant thirst and dry mouth may be caused by diabetes, uremia, hyperthyroidism and other diseases. Diabetes mellitus is mainly a blood test, uremia is mainly a urine measurement test, hyperthyroidism requires physical examination and thyroid function test. 1. Diabetes: Diabetes examination is mainly blood sampling for relevant examinations, including blood glucose measurement, glucose tolerance test, glycated hemoglobin measurement, glycated plasma protein measurement, pancreatic β-cell function examination, and so on. Diabetes can cause patients to appear thirsty, so the constant thirst and dry mouth may be caused by diabetes. 2. Uremia: It can be diagnosed by urine volume monitoring, blood osmolality, urine osmolality, electrolytes, renal tubular function, urine specific gravity, pituitary MRI, renal ultrasound and other tests to diagnose whether it is uremia caused by increased osmotic pressure of the body caused by thirst, and to make a diagnosis. 3. Hyperthyroidism: A physical examination will first be performed to find out if there are any abnormal signs. Afterwards, the patient may be advised to undergo thyroid function tests and thyroid imaging. Hyperthyroidism may cause the patient to have a high cellular metabolism and therefore symptoms of thirst. If the symptoms of constant thirst and dry mouth persist without relief, it is necessary to seek medical attention and follow the doctor’s instructions for examination.