Causes of abnormal weight loss

There are many causes of abnormal weight loss including:1

Loss of appetite and conditions that prevent food consumption, e.g. dysphagia, painful mouth sores, newly applied orthodontic appliances, or loss of teeth.
Persistent vomiting, e.g. pyloric stenosis, hiatus hernia.
Malabsorption, e.g. coeliac disease, chronic pancreatitis, Crohn's disease, gastrointestinal infection, gastrointestinal fistulas, carcinoid, intestinal hypermotility, hepatobiliary disease, food intolerance.
Medication, especially polypharmacy in the elderly.
Endocrine: diabetes mellitus, hyperthyroidism, Addison's disease, gut hormone tumours (e.g. VIPoma).
Malignancy: lymphoma, leukaemia, carcinoma, sarcoma.
Systemic disease, e.g. heart failure, chronic respiratory disease, chronic renal failure, liver failure, rheumatoid arthritis, systemic lupus erythematosus.
Acute infection.
Chronic infections and infestations, e.g. tuberculosis, HIV (one in ten in one study),2 parasitic infections.
Drug abuse, heavy smoking.
Malnutrition, social isolation.
Psychological: stressful life events, depression, anorexia nervosa, psychoses, manipulative behaviour, food phobias.
Presentation
Patients may realise themselves that they have lost weight or this may be brought to their attention by friends or family.
A clinician may note that the patient has dramatically lost weight or notice that their clothing is loose-fitting.
The clinical assessment includes both consideration of possible physical causes as well as careful evaluation of possible psychological causes such as depression. It is very important to avoid inappropriate, unnecessary and potentially harmful investigations.
The presentation will depend on the underlying cause.
A thorough history and examination are essential in establishing the underlying cause and identifying appropriate investigations.
Associated symptoms may include:
Gastrointestinal symptoms.
Lethargy, weakness.
An underlying condition, e.g. respiratory, neuromuscular.
Alcohol or drug abuse.
Dementia: mental state assessment may be indicated.