What hormone is secreted by the hypothalamus when blood pressure decreases or ionic concentration increases, which results in increased reabsorption of water in the nephron?

Antidiuretic hormone

Vasopressin, also named antidiuretic hormone (ADH), arginine vasopressin (AVP) or argipressin,[3] is a hormone synthesized as a peptide prohormone in neurons in the hypothalamus, and is converted to AVP. It then travels down the axon of that cell, which terminates in the posterior pituitary, and is released from vesicles into the circulation in response to extracellular fluid hypertonicity (hyperosmolality). AVP has two primary functions. First, it increases the amount of solute-free water reabsorbed back into the circulation from the filtrate in the kidney tubules of the nephrons. Second, AVP constricts arterioles, which increases peripheral vascular resistance and raises arterial blood pressure.[4][5][6]