Gastroenteritis: This is often the most common cause of severe dehydration in adults, resulting from viral or bacterial infections that lead to excessive fluid loss through vomiting and diarrhea.

Food Poisoning: Similar to gastroenteritis, food poisoning can cause severe vomiting and diarrhea, leading to dehydration.
Diabetic Ketoacidosis (DKA): Although more common in individuals with known diabetes, new-onset diabetes can present with DKA, characterized by hyperglycemia, metabolic acidosis, and dehydration.
Heat Stroke: Especially relevant in hot climates or during heatwaves, heat stroke can cause dehydration due to excessive sweating and altered mental status.
Cholera: In areas where cholera is endemic, this bacterial infection can cause severe, watery diarrhea leading to rapid dehydration.

Septic Shock: Although less common, septic shock is a life-threatening condition that can present with dehydration among other symptoms. Early recognition and treatment are crucial.
Adrenal Insufficiency: This condition, including Addisonian crisis, can cause dehydration due to the body's inability to regulate fluids and electrolytes properly.
Pyloric Stenosis or Intestinal Obstruction: These conditions can lead to severe vomiting and subsequent dehydration, requiring prompt surgical intervention.

Hemolytic Uremic Syndrome (HUS): A condition characterized by hemolytic anemia, acute kidney injury, and thrombocytopenia, often following a diarrheal illness caused by E. coli O157:H7.
Pituitary Apoplexy: A rare endocrine emergency that can lead to acute adrenal insufficiency and dehydration among other symptoms.
Malignancy-related Dehydration: Certain cancers, especially those affecting the gastrointestinal tract or causing hypercalcemia, can lead to dehydration through various mechanisms.