Depression: Often presents with unintentional weight loss due to decreased appetite and interest in food. It's a common condition that can have significant physical manifestations, including weight loss.

Diabetes Mellitus: Uncontrolled diabetes can lead to weight loss due to the body's inability to use glucose for energy, resulting in the breakdown of fat for fuel.
Hyperthyroidism: An overactive thyroid gland can increase metabolism, leading to unintentional weight loss despite increased appetite.
Chronic Obstructive Pulmonary Disease (COPD): Advanced COPD can cause weight loss due to increased energy expenditure associated with breathing and decreased appetite.
Gastrointestinal Disorders (e.g., Celiac Disease, Crohn's Disease): Conditions that affect the absorption of nutrients can lead to weight loss.

Cancer: Various types of cancer can cause unintentional weight loss, especially in the absence of other symptoms. It's crucial to consider cancer in the differential diagnosis due to its potential severity and the importance of early detection.
AIDS: Acquired Immunodeficiency Syndrome can lead to weight loss due to chronic infection, diarrhea, and decreased appetite.
Tuberculosis: This infectious disease can cause significant weight loss, especially if left untreated.

Addison's Disease: A rare endocrine disorder that can lead to weight loss due to the adrenal glands not producing enough cortisol and aldosterone.
Pheochromocytoma: A rare tumor of the adrenal gland that can cause weight loss, among other symptoms like hypertension and palpitations.
Whipple's Disease: A rare, systemic bacterial infection that can lead to weight loss, diarrhea, and malabsorption.