Vasopressin
Vasopressin (ADH, AVP) is the hormone which controls the production of urine in the bladder. It is secreted by the pituitary gland based on the salt concentration of the blood. It is negative feedback: the more concentrated the salt in the blood is, the more vasopressin is secreted, which in turn makes the kidney tubules more permeable, allowing more water to be reabsorbed into the blood and creating less, but more concentrated, urine. It also triggers feelings of thirst.