Depression: Often presents with sudden 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: Especially type 1 diabetes, can cause sudden weight loss due to the body's inability to use glucose for energy, leading to the breakdown of fat for fuel.
Hyperthyroidism: An overactive thyroid gland can increase metabolism, leading to weight loss despite increased appetite.
Gastrointestinal Disorders: Conditions like celiac disease, Crohn's disease, or ulcerative colitis can lead to malabsorption and weight loss.
Chronic Infections: Such as tuberculosis (TB), can cause weight loss due to increased metabolic demand and decreased appetite.

Cancer: Various types of cancer, especially pancreatic, lung, and gastrointestinal cancers, can cause unexplained weight loss. Missing a diagnosis of cancer can be fatal.
AIDS: Acquired Immunodeficiency Syndrome can lead to weight loss due to chronic infection, malabsorption, and increased metabolic rate.
Pulmonary Tuberculosis: A serious infection that can cause significant weight loss if not treated promptly.

Addison's Disease: A rare endocrine disorder where the adrenal glands do not produce enough cortisol and aldosterone, leading to weight loss among other symptoms.
Pheochromocytoma: A rare tumor of the adrenal gland that can cause weight loss due to increased metabolism.
Whipple's Disease: A rare, systemic bacterial infection that can cause malabsorption and weight loss.
Amyotrophic Lateral Sclerosis (ALS): A progressive neurological disease that can lead to weight loss due to difficulty swallowing and malnutrition.