Which hormone is responsible for water balance and concentration in urine?

Antidiuretic hormone (ADH), also known as vasopressin, plays a crucial role in regulating water balance within the body. Its primary function is to promote the reabsorption of water in the kidneys. When ADH is released, it acts on the collecting ducts of the nephrons, increasing their permeability to water and allowing more water to be reabsorbed back into the bloodstream. This process leads to a decrease in urine volume and an increase in the concentration of urine, thereby helping to maintain the body's fluid balance.

In contrast, while aldosterone does influence water balance, its primary function is to regulate sodium and potassium levels, which indirectly affects water retention due to osmotic forces. Thyroxine is a thyroid hormone that primarily regulates metabolism, and cortisol is a glucocorticoid involved in stress response and metabolism but does not have a direct role in controlling water balance in urine. Thus, ADH is the hormone specifically responsible for the regulation of water balance and urine concentration.