Vasopressin (ADH): Water Balance

Vasopressin, also called antidiuretic hormone (ADH), is the body's main signal for conserving water. It tells the kidneys how much fluid to hold on to and how much to release as urine, helping keep blood concentration and volume within a narrow, healthy range.

What vasopressin is

Vasopressin is a small protein (peptide) hormone built from a short chain of nine amino acids. Its older clinical name, antidiuretic hormone, describes its central job: reducing the amount of water lost in urine ("antidiuretic" means working against the loss of water). At higher concentrations it can also tighten blood vessels, which is where the name "vasopressin" comes from. In everyday physiology, its water-conserving role is the one that matters most, and the two names refer to exactly the same molecule.

Vasopressin acts by attaching to specific receptors on target cells. Different receptor types explain its different jobs: one type, found in the kidney, drives water reabsorption, while another type, found on blood vessels, drives the narrowing of vessels. This is why the same hormone can have more than one effect depending on where it lands.

Where it is produced

Vasopressin is made by nerve cells (neurons) in the hypothalamus, a control center deep in the brain. Unlike many hormones, it is not released near where it is made. Instead it travels down the long fibers of these neurons and is stored in the posterior (back) part of the pituitary gland, just beneath the brain. The pituitary then releases it into the bloodstream when the body needs to conserve water. Because vasopressin depends on this whole pathway — production in the hypothalamus, transport down the nerve fibers, storage and release from the posterior pituitary, and a kidney able to respond — a problem at any point along the route can disturb water balance.

What it does across body systems

Kidneys and water balance

The kidney is vasopressin's main target. It prompts the kidney's collecting ducts to reabsorb water back into the blood rather than losing it in urine, producing a smaller volume of more concentrated urine. This is the single most important action of the hormone and the basis for its antidiuretic name.

Blood concentration

By fine-tuning how much water the kidneys keep, vasopressin helps hold the concentration of the blood — its saltiness, measured as osmolality — within a narrow range. Sodium is the main contributor to this concentration, so vasopressin's water handling and the blood's sodium level are closely linked.

Blood vessels and pressure

At higher concentrations vasopressin causes blood vessels to narrow, which can help support blood pressure during significant fluid or blood loss. In ordinary day-to-day conditions this effect is minor; it becomes more relevant in situations of severe volume loss.

How levels are regulated

Vasopressin release is governed mainly by the concentration of the blood. Specialized sensors in the brain, called osmoreceptors, detect when the blood becomes too concentrated — for example after sweating heavily, eating salty food, or not drinking enough. When they sense rising concentration, the hypothalamus signals the posterior pituitary to release more vasopressin, the kidneys hold on to water, and the blood is diluted back toward normal. When the blood becomes too dilute, vasopressin release falls and the kidneys pass more water as urine. This is a sensitive, continuous feedback loop that operates around the clock.

A large drop in blood volume or blood pressure can also trigger release, even when blood concentration is normal, because maintaining circulation takes priority. Thirst works alongside this hormone system: the same signals that raise vasopressin also tend to make a person feel thirsty, so behavior (drinking) and hormone action (conserving) work together to restore balance. Other factors, including nausea and certain medications, can also influence release.

What high or low levels can be associated with

When vasopressin activity is too low, or the kidneys cannot respond to it, the body may lose large amounts of dilute urine and the person can feel very thirsty and drink a great deal to keep up. This pattern is seen in a condition historically called diabetes insipidus — a name that is unrelated to the far more common, sugar-related diabetes mellitus, even though both can cause frequent urination. The problem can arise either because too little vasopressin is made (a central cause) or because the kidney does not respond to it (a kidney-based cause).

When vasopressin is present in excess relative to the body's needs, the body retains too much water, which can dilute the blood and lower sodium levels. A markedly low blood sodium can cause symptoms ranging from headache and confusion to more serious effects, which is why this pattern is taken seriously. Both extremes are qualitative descriptions here; the underlying causes vary widely and are evaluated by clinicians. See the conditions index for related topics.

How it is measured

Vasopressin itself is unstable in a blood sample and difficult to measure reliably, so it is not part of routine blood panels. When a problem with water balance is suspected, clinicians more often assess the situation indirectly — by measuring blood sodium, blood and urine concentration (osmolality), and urine volume, sometimes alongside a supervised water-deprivation or related test that observes how the body responds. A more stable marker called copeptin, which is released together with vasopressin, is used in some specialized settings as a practical stand-in. For general context on testing, see the blood tests overview and the glossary.

How it relates to other hormones

Vasopressin is one of two hormones stored and released by the posterior pituitary; the other is oxytocin, a closely related peptide also made in the hypothalamus. Both are short amino-acid chains that differ by only a small part of their structure, yet they have very different roles. Vasopressin also works alongside the body's salt-regulating hormones, particularly aldosterone from the adrenal gland: vasopressin chiefly manages water, while aldosterone chiefly manages sodium, and together they keep the volume and concentration of body fluids in balance. Because these systems overlap, water and salt handling are best understood as a coordinated whole rather than as separate processes.