The primary trigger for ADH (antidiuretic hormone) release from the hypothalamus is high blood osmolarity. When blood osmolarity rises, osmoreceptors in the hypothalamus detect this change, stimulating the posterior pituitary gland to secrete ADH. ADH then acts on the kidneys to promote water reabsorption, reducing urine output and diluting the blood plasma to restore normal osmolarity. This mechanism is critical for maintaining fluid balance and electrolyte homeostasis. While low blood pressure and sodium concentration can also influence ADH release, the most direct and sensitive trigger is elevated osmolarity.

Reference: Guyton and Hall Textbook of Medical Physiology, Guyton & Hall, 14th Edition.