Other causes of unintentional weight loss include: 1  cancer. 2  hyperthyroidism (overactive thyroid gland). 3  hypothyroidism (underactive thyroid gland). 4  abdominal infection.5  gastroenteritis. 6  malnutrition. 7  dementia. 8  Celiac disease.9  HIV/AIDS.ther causes of unintentional weight loss include: 1  cancer. 2  hyperthyroidism (overactive thyroid gland). 3  hypothyroidism (underactive thyroid gland). 4  abdominal infection. 5  gastroenteritis. 6  malnutrition. 7  dementia. 8  Celiac disease. 9  HIV/AIDS.