Hint: Is made of special nerve cells which are found in an area at the base of the brain which is called the hypothalamus. The nerve cells transport the hormone down their nerve fibres to the pituitary gland where the hormone is released into the bloodstream.
Complete answer: It is an antidiuretic hormone or vasopressin which helps in maintaining ionic balance in the body. Antidiuretic hormone helps to control blood pressure by acting on the kidneys and the blood vessels. Its most important role is to conserve the fluid volume of your body by reducing the amount of water passing out in urine. It allows the water back from the urine to the body from the kidney. Thus, more water returns into the blood streams, which makes the concentration of urine increase and reduces the loss of water. When the bloodstreams constrict, it leads to high pressure in blood streams, thus making higher concentration of vasopressin. Deficiency of water in the body is restored by increasing water intake. If a high level of Adh is present, it will cause the kidney to retain body form. Low level of ADH could be a signal of damage to the Hypothalamus. It also represents an effort by the kidneys to excrete out the extra water from the body.
Note: There is a condition, where excess of ADH is released when it is not needed it is called hyponatraemia. Because of this condition excessive water retention dilutes the blood, giving a feature of low concentration.