Noticeable weight loss caused by stress is not a matter of concern, as it is temporary and weight is regained once you start feeling happy and stress-free. However, weight loss that is not caused by stress or is unplanned is a matter of concern, as it may suggest an underlying disease symptom that might require immediate medical attention.

There are various causes of weight loss; however, in cases of unplanned weight loss, the exact cause cannot always be identified. The most common causes of weight loss include:
- Depression
- Hyperthyroidism (in which the thyroid glands are overactive)
- Underactive thyroid glands treated extensively without monitoring regularly
- Cancer

Following are some of the rare causes of unexpected weight loss:
- Medication side effects
- Infection
- Chronic inflammatory conditions (rheumatoid arthritis)
- Dementia
- Gut-related problems (ulcers)
- Disorders leading to difficulties in swallowing
- Alcohol or drug misuse
- Dental issues (mouth ulcers or loss of teeth)

Yes, when there is weight loss of more than 5% of the body weight in 6-12 months, it is a matter of concern, as it can be considered a sign of an underlying medical condition or significant malnutrition.

To diagnose the cause of unexplained weight loss, physical examination along with the analysis of the following information is carried out by a physician:
- The extent of weight loss and the period over it occurred
- Recent disease, vomiting or diarrhoea, medication use or stress
- Change in lifestyle, including eating pattern, excessive intake of water or alcohol consumption