Antidiuretic hormone

Vasopressin, also known an antidiuretic hormone (ADH), arginine vasopressin (AVP), or argipressin is a hormone that causes the kidneys to reabsorb water and return it to the circulation, thus controling the concentration of urine.

ME/CFS
According to Dr. David Kaufman, formerly of the Open Medicine Institute, diagnostic biomarkers for ME/CFS include low natural killer cell function, low vasopressin levels, mutations of the MTHFR gene, and abnormalities of the hypothalamic-pituitary-adrenal axis (HPA).