Polyuria

Polyuria is urine output of > 3 L/day; it must be distinguished from urinary frequency, which is the need to urinate many times during the day or night but in normal or less-than-normal volumes. Either problem can include nocturia.

Pathophysiology

Water homeostasis is controlled by a complex balance of water intake (itself a matter of complex regulation), renal perfusion, glomerular filtration and tubular reabsorption of solutes, and reabsorption of water from the renal collecting ducts.
When water intake increases, blood volume increases and blood osmolality decreases, decreasing release of ADH (also referred to as argininevasopressin) from the hypothalamic-pituitary system. Because ADH promotes water reabsorption in the renal collecting ducts, decreased levels of ADH increase urine volume, allowing blood osmolality to return to normal.
Additionally, high amounts of solutes within the renal tubules cause a passive osmotic diuresis (solute diuresis) and thus an increase in urine volume. The classic example of this process is the glucose-induced osmotic diuresis in uncontrolled diabetes mellitus, when high urinary glucose levels (> 250 mg/dL) exceed tubular reabsorption capacity, leading to high glucose levels in the renal tubules; water follows passively, resulting in glucosuria and increased urine volume.
Therefore, polyuria results from any process that involves
Sustained increase in water intake (polydipsia)
Decreased ADH secretion (central diabetes insipidus)
Decreased peripheral ADH sensitivity (nephrogenic diabetes insipidus)
Solute diuresis

Etiology

The most common cause of polyuria in adults is

Taking diuretics

The most common cause of polyuria  in both adults and children is
Uncontrolled diabetes mellitus
In the absence of diabetes mellitus, the most common causes are
Primary polydipsia
Central diabetes insipidus
Nephrogenic diabetes insipidus