The reasons why diabetic patients suddenly become thin include the following: First, unsatisfactory blood sugar control, when the blood sugar of diabetic patients rises significantly, a large amount of glucose is excreted from the urine, the body carries out fat decomposition will appear the symptoms of lethargy and overeating. Second, hyperthyroidism, typical hyperthyroidism manifested as fear of heat, excessive sweating, wasting, increased frequency of stools, overeating, easy hunger and other symptoms. Diabetic patients suddenly become thin, should exclude the cause of hyperthyroidism. Third, malignant tumors, when diabetic patients combined with malignant tumors and other wasting diseases, can appear the symptoms of sudden weight loss. In diabetic patients, the incidence of pancreatic cancer and other tumors is also higher than in non-diabetic patients. Therefore, sudden weight loss in diabetic patients should also be considered as a tumor factor. Therefore, if diabetic patients suddenly become thin, they should check the control of blood sugar and exclude the diagnosis of hyperthyroidism and malignant tumors.