Illnesses and disorders are a major cause of unexplained or unintentional weight loss. The reasons for the weight loss vary. Loss of appetite (anorexia) is often involved and may be due to a wide range of factors, such as a general ill feeling (malaise), decreased activity levels, pain or hypercalcemia (excessive calcium in the blood, caused by disorders including sarcoidosis and hyperparathyroidism). However, some metabolic disorders, notably diabetes and hyperthyroidism, can cause a person to overeat but still lose weight.

Another common reason for weight loss is malnutrition. The many causes of malnutrition include intestinal obstructions, neglect of dependent individuals by caregivers, impaired swallowing (dysphagia) due to conditions such as a stroke or Parkinson's disease, or difficulty eating due to conditions such as severe rheumatoid arthritis in the hands.

Nausea and vomiting may also play a role. The many causes of nausea and vomiting range from migraines to pregnancy to pancreatitis to chemotherapy.

Conditions associated with weight loss include:

• Many cancers, including colorectal, stomach, esophageal, pancreatic and lung, and cancer treatments including chemotherapy, radiation therapy, surgery and some types of hormonal or biological therapy .
  
  
• Other gastrointestinal and digestive problems, including chronic diarrhea, colitis, food intolerances, ulcers and some liver and gallbladder diseases .
  
  
• Depression .
  
  
• Eating disorders, including bulimia and anorexia nervosa .
  
  
• Endocrine disorders including diabetes, forms of hyperthyroidism (overactive thyroid gland) such as Graves' disease, forms of hyperadrenalism (underactive adrenal glands) such as Addison's disease, and hyperparathyroidism (overactive parathyroid glands).
  
  
• Infectious conditions, such as AIDS, tuberculosis or parasitic infection.
  
  
• Pain conditions. Examples include chronic fatigue syndrome, lupus, migraines or other severe headaches, myositis, polymyalgia rheumatica, sarcoidosis, temporal arteritis, TMJ disorder and certain forms of arthritis, such as osteoarthritis of the jaw or ankylosing spondylitis.
  
  
• Neurological conditions, such as amyotrophic lateral sclerosis (ALS) and Parkinson's disease.
  
  
• Use of certain medications, including amphetamines, laxatives, some diabetes drugs (e.g., incretins, biguanides) and some thyroid drugs.
  
  

Several recent studies have found that unexplained weight loss may precede Alzheimer's disease or other forms of dementia.

Nonmedical factors also may cause unintentional weight loss. Children and others sometimes lose weight as a tactic to gain attention or sympathy. Some smokers find that tobacco use reduces their appetite.

Temporary factors also may cause unintentional weight loss. For example, people with mouth sores or acute and chronic dental problems often shy away from eating, resulting in weight loss that lasts until their oral symptoms improve. Illnesses such as the flu or common cold may temporarily reduce appetite and cause weight loss.