ANTIDIURETIC HORMONE (ADH)
Produced in the hypothalamus and released by the posterior pituitary gland, antidiuretic hormone (ADH) controls the permeability of the collecting ducts to water. Increased levels of ADH result in greater reabsorption and decreased water excretion.

When blood volume is reduced as a result of decreased fluid intake or injury, the body can become dehydrated. Concentration of salts dissolved in the blood increases, causing a rise in osmotic pressure. Receptors in the hypothalamus react to the shift in osmotic pressure and trigger the posterior lobe of the pituitary to activate more ADH. At the same time, a thirst center in the hypothalamus responds by stimulating a feeling of thirst.

Conversely, when an abundance of water is consumed, the blood becomes less concentrated and osmotic pressure decreases. The pituitary gland compensates by reducing ADH release, causing the quantity of water reabsorbed from the collecting ducts to decrease. The retained water causes the kidneys to produce an increased volume of dilute urine.

A condition called diabetes insipidus results when the pituitary gland does not produce enough ADH, or from an acquired insensitivity of the kidney to ADH. Water is inadequately reabsorbed from the collecting ducts, so a large quantity of urine is produced. Diabetes insipidus often can be managed clinically by ADH injections or by ADH nasal spray treatments.