Final answer:
Antidiuretic hormone (ADH) regulates kidney retention of water by making the collecting ducts in the kidneys more permeable to water, allowing water to be reabsorbed from urine back into the bloodstream.
Step-by-step explanation:
The pituitary hormone that regulates kidney retention of water is Antidiuretic hormone (ADH). ADH is produced by the hypothalamus and released from the posterior pituitary gland. Its main function is to regulate the amount of water excreted by the kidneys by making the collecting ducts in the kidneys more permeable to water, allowing water to be reabsorbed from urine back into the bloodstream. This helps in conserving water and maintaining the body's fluid balance.