Antidiuretic hormone (ADH) is a hormone that helps regulate the amount of water in your body. It is produced by the hypothalamus, an area at the base of your brain, and stored in the posterior pituitary gland. ADH works to control the amount of water your kidneys reabsorb as they filter out waste from your blood. When ADH makes it to your kidneys, it signals them to conserve water and produce more concentrated urine. The amount of ADH in your body is regulated by sensors that detect changes in your blood volume and communicate with your brain and pituitary gland.

Too much ADH can cause a condition called the syndrome of inappropriate antidiuretic hormone (SIADH), where ADH is released into your blood sporadically, sometimes releasing far more than your body needs. This can cause excessive water retention, diluting the blood and leading to low salt concentration.

ADH testing alone won’t diagnose your condition, but it can help distinguish between central diabetes insipidus, nephrogenic diabetes insipidus, and SIADH. If you have low levels of ADH, you can replace it by taking a synthetic form of the hormone in pill form. Your doctor may also suggest drinking more water and making changes to your diet and lifestyle.