Osmoreceptors in the hypothalamus detect changes in plasma osmolality, primarily the concentration of sodium. When osmolality rises, these receptors stimulate the hypothalamic neurosecretory cells to increase the secretion of antidiuretic hormone (ADH) from the posterior pituitary. ADH then acts on the kidneys to promote water reabsorption, reducing plasma osmolality and restoring fluid balance. This mechanism is crucial for maintaining body fluid homeostasis. Thus, the primary function of hypothalamic osmoreceptors is to regulate ADH secretion in response to osmotic changes.

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