Unexplained weight loss is considered a symptom. When you lose weight without trying any weight loss methods like dieting or exercising in any manner, it is considered a symptom. Loss of more than 5% of your body weight should prompt you to see a doctor.

Causes of weight loss can be medical and non-medical. Medical reasons include ulcer, diabetes, cancer, AIDS, Parkinson's disease, hyperactivity of thyroid gland, depression, COPD, drug abuse, depression, tuberculosis & heart failure. Non-medical causes can be a change of diet, stress, excessive smoking and alcohol consumption.

Severe autoimmune disorders like lupus and rheumatoid arthritis; digestive tract disturbances like gluten allergy, lactose intolerance, irritable bowel syndrome, and inflammatory bowel diseases, are also known to cause weight loss.

Yes, on an average, losing 5 percent of one body weight in 6-12 months is considered unhealthy and cause for concern. One must visit a doctor.

The most important step to diagnose the cause for weight loss is to take a detailed medical history as to when did it start, over how much time has the weight loss occurred, etc. Physical examination is also needed. Certain blood tests and imaging scans might be needed to identify the exact cause for unexplained weight loss.