Definition

Syndrome of inappropriate antidiuretic hormone secretion (SIADH) is a condition in which the body makes too much antidiuretic hormone (ADH). ADH is also called vasopressin. This hormone helps the kidneys control the amount of water your body loses through the urine. SIADH causes your body to retain too much water.

ADH is a substance produced naturally in an area of the brain called the hypothalamus. It is then released by the pituitary gland at the base of the brain.

Alternative Names

SIADH; Inappropriate secretion of antidiuretic hormone; Syndrome of inappropriate ADH release; Syndrome of inappropriate antidiuresis

Causes

There are many reasons why your body may make a lot of ADH. Common situations when ADH is released into the blood when it should not be produced (inappropriate) include:

• Medicines, such as certain seizure medicines, antidepressants, cancer medicines, opiates (less often), and heart, diabetes, and blood pressure medicines
• Surgery under general anesthesia
• Disorders of the brain, such as injury, infections, and stroke
• Brain surgery in the region of the hypothalamus
• Tuberculosis, cancer, chronic infections, and lung disease, such as pneumonia
• Substance use disorder

Rare causes include:

• Rare diseases of the hypothalamus or pituitary
• Leukemia and cancer of the small intestine, pancreas, and brain
• Mental disorders

Symptoms

With SIADH, the urine is very concentrated. Not enough water is excreted and there is too much water in the blood. This dilutes many substances in the blood such as sodium. A low blood sodium level is the most common cause of symptoms of too much ADH. It is also the most common clue that a person may have SIADH.

Often, there are no symptoms from a mildly low sodium level. More symptoms occur the lower the level of the sodium.

When symptoms do occur, they may include any of the following:

• Nausea and vomiting
• Headache
• Problems with balance that may result in falls
• Mental changes, such as confusion, memory problems, strange behavior
• Seizures or coma, in severe cases

Exams and Tests

Your health care provider will perform a complete physical examination to help determine the cause of your symptoms. Establishing fluid status (dehydrated, volume overload, normal volume) is an important part of making the correct diagnosis.

Lab tests that can confirm and help diagnose low sodium include:

• Comprehensive metabolic panel (includes blood sodium)
• Osmolality blood test
• Urine osmolality
• Urine sodium and potassium
• Toxicology screens for certain medicines
• Lung and brain imaging tests in children suspected of having SIADH

ADH can be measured in the blood, but is difficult to process for many labs and takes up to two weeks to get a result. Serum co-peptin can be used to estimate the ADH level in the blood.

Treatment

Treatment depends on the cause of the problem. For example, surgery is done to remove a tumor producing ADH. Or, if a medicine is the cause, its dosage may be changed or another medicine may be tried.

In all cases, the first step is to limit fluid intake. This helps prevent excess fluid from building up in the body. Your provider will tell you what your total daily fluid intake should be. The restriction is not just for water, but for almost all fluids (coffee, tea, juice, soda, etc.).

If you have severe symptoms, it is a medical emergency. This is usually treated with salt solution (3% saline) given through an IV into the veins (intravenous) in the hospital.

Medicines may be needed to block the effects of ADH on the kidneys so that excess water is excreted by the kidneys. These medicines may be given as pills or as injections given into the veins.

Outlook (Prognosis)

Outcome depends on the condition that is causing the problem. Low sodium that occurs rapidly, in less than 48 hours (acute hyponatremia), is more dangerous than low sodium that develops slowly over time. When the sodium level falls slowly over days or weeks (chronic hyponatremia), the brain cells have time to adjust and the acute symptoms such as brain swelling don't occur. Chronic hyponatremia is associated with nervous system problems such as poor balance and poor memory. Many causes of SIADH are reversible. Rapid correction of chronic hyponatremia can also cause dangerous complications (osmotic demyelination).

Possible Complications

In severe cases, low sodium can lead to:

• Decreased consciousness, hallucinations or coma
• Brain herniation
• Death

When to Contact a Medical Professional

When your body's sodium level drops too much, it can be a life-threatening emergency. Contact your provider right away if you have symptoms of this condition.

References

Pashankar R. Endocrinology. In: Anderson CC, Kapoor S, Mark TE, eds. The Harriet Lane Handbook. 23rd ed. Philadelphia, PA: Elsevier; 2024:chap 10.

Thompson CJ, Garrah A. Disorders of sodium, diabetes insipidus and hyponatremia. In: Robertson RP, ed. DeGroot's Endocrinology. 8th ed. Philadelphia, PA: Elsevier; 2023:chap 12.

Verbalis JG. Disorders of water balance. In: Yu ASL, Chertow GM, Luyckx VA, Marsden PA, Skorecki K, Taal MW, eds. Brenner and Rector's The Kidney. 11th ed. Philadelphia, PA: Elsevier; 2020:chap 15.