The different causes of unexplained weight loss include cancer, thyroid problems, diabetes, chronic obstructive pulmonary disease, depression, digestive disorders, AIDS and hypercalcemia, notes Mayo Clinic. Some other causes can be parasitic infections that cause diarrhea, drug abuse and an eating disorder. In most cases, an unexplained weight loss may be due to an underlying and undiagnosed medical condition.
Unexplained weight loss can require a medical evaluation if the patient loses more than 5 percent of his weight in a time span of 6 to 12 months, states Mayo Clinic. Additionally, if a patient notices any unusual symptoms accompanying the weight loss, he should see a doctor, notes the National Institutes of Health's MedlinePlus.
Different types of gastrointestinal disorders can cause weight loss, including peptic ulcers, ulcerative colitis and gastroenteritis. Similarly, Crohn's disease and celiac disease, which also affect the digestive system, may cause weight loss. Conditions that affect the thyroid, such as hyperthyroidism (overactive thyroid) or hypothyroidism (underactive thyroid), may lead to weight loss as well.
Although cancer symptoms can be different depending on its type and locations, the first symptom of cancer may be weight loss of more than 10 pounds, states the American Cancer Society. Unexplained weight loss also can be associated with cancer that develops in areas such as the lungs, pancreas and stomach. However, there can be other symptoms associated with cancer, including feeling tired, fever and pain in the affected area.