In response to increased tonicity (principally caused by an increase in sodium concentration), osmoreceptors in the hypothalamus respond (similar to the response of baroreceptors to decreased effective circulating volume) by stimulating ADH release (causing water retention in the kidneys without sodium) and thirst (water gain without sodium). This will dilute out the sodium, decreasing tonicity. Conversely, with decreased tonicity (decreased effective osmolality), usually due to hyponatremia, osmoreceptors in the hypothalamus inhibit ADH secretion (promoting water loss) and thirst (preventing water gain), which serves to increase sodium concentrations.
