There are more reasons for sudden weight loss, such as diabetes, hyperthyroidism, tuberculosis, tumors and so on. 1. Diabetes mellitus: it is a group of metabolic diseases characterized by chronic hyperglycemia caused by multiple etiological factors, which can lead to the patient to appear three more and one less typical symptoms, that is, polyuria, polydipsia, polyphagia, and weight loss. So diabetic patients can also appear suddenly thin phenomenon. 2. Hyperthyroidism: also known as hyperthyroidism, is mainly caused by certain factors that lead to overactive thyroid function, which causes excessive secretion of thyroid hormones released into the bloodstream, which can lead to agitation, easy to be hungry, hyperphagia, and weight loss in non-dieting situations. 3. Tuberculosis: It is a chronic infectious disease caused by Mycobacterium tuberculosis, which can involve many organs of the body, mainly with pulmonary tuberculosis being more common. It can lead to low fever in the afternoon, fatigue and weight loss. Therefore, sudden weight loss may also be caused by tuberculosis. 4. Tumor: malignant tumors in the late stage lead to malignant disease, which makes patients become thin rapidly. In addition, chronic diarrhea, peptic ulcer, etc., can cause patients to suddenly become thin, it is best to go to the hospital in time to improve the examination for diagnosis.