Part of that extra thickness is caused by the protein keratin, which is nourished in deeper layers of the skin but forms a surface layer of dead cells on the epidermis. This extra thickness may also explain why your hands and feet become wrinkled when you soak in the tub.
Antidiuretic hormone (ADH), also called arginine vasopressin (AVP), is a hormone that helps regulate water balance in the body by controlling the amount of water the kidneys reabsorb while they are filtering wastes out of the blood. This test measures the amount of ADH in the blood.
The hypothalamus sends a message to the pituitary, which releases ADH. This travels in the blood to the kidneys, which save more water, so that the urine becomes less in volume and more concentrated. The level of water in the blood can then begin to rise as water is ingested and produced by the process of respiration.