Polydipsia is abnormally intense, persistent thirst driving excessive fluid intake. Physiologically, thirst guards water balance tightly, so polydipsia signals that something is wrong: the classic cause is diabetes mellitus, where high blood glucose spills into urine, dragging water with it in osmotic diuresis; diabetes insipidus, where the kidneys cannot concentrate urine; or kidney disease. Certain medications, dry mouth conditions, and psychiatric polydipsia in some mental illnesses also cause it. Clinicians treat new-onset polydipsia, especially with polyuria and weight loss, as a red flag for diabetes until proven otherwise, checking blood glucose and HbA1c. Management addresses the underlying disorder while ensuring the excessive intake does not dilute blood sodium dangerously, a particular risk when thirst outruns the kidneys’ excretory capacity.