Syndrome of Inappropriate Antidiuretic Hormone (SIADH)

What is Syndrome of Inappropriate Antidiuretic Hormone (SIADH)?

Syndrome of Inappropriate Antidiuretic Hormone, or SIADH, happens when your body makes too much antidiuretic hormone. This hormone is also called ADH or vasopressin. It tells your kidneys to hold onto water instead of releasing it through urine.

When you have SIADH, your kidneys retain too much water. This dilutes the sodium in your blood. Your sodium level drops below 135 mEq/L, a condition called hyponatremia. Meanwhile, your urine stays concentrated even though your blood is already diluted. This creates an imbalance that can affect your brain, muscles, and cells throughout your body.

SIADH is not a disease on its own. It is usually a sign of an underlying problem. Lung infections, certain cancers, brain injuries, and some medications can all trigger excess ADH production. Catching low sodium early through blood testing helps prevent serious complications and guides treatment of the root cause.

Symptoms

Nausea and vomiting
Headache
Confusion or difficulty concentrating
Fatigue and low energy
Muscle weakness or cramps
Loss of appetite
Irritability or mood changes
Seizures in severe cases
Restlessness or unusual behavior
Loss of consciousness in extreme cases

Many people with mild SIADH have no symptoms early on. Symptoms usually appear when sodium drops below 125 mEq/L. Severe symptoms typically occur when sodium falls below 115 mEq/L or drops very quickly. This makes regular blood testing important for people at risk.

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Causes and risk factors

SIADH develops when something triggers your body to make too much ADH. Lung conditions are common triggers, including pneumonia, tuberculosis, and small cell lung cancer. Brain problems like meningitis, stroke, head trauma, and brain tumors can also cause excess ADH. Certain medications stimulate ADH release, including some antidepressants, pain medications, and cancer drugs. Surgery and stress can temporarily raise ADH levels as well.

Some cancers produce ADH on their own, especially lung cancer and certain blood cancers. Chronic conditions like HIV and hypothyroidism increase risk. Older adults are more vulnerable because they have less ability to regulate fluid balance. Genetic factors can make some people more sensitive to ADH. Understanding your risk factors helps you and your doctor watch for early signs through regular blood testing.

How it's diagnosed

SIADH is diagnosed through blood and urine tests that show a specific pattern. Your blood will show low sodium, typically below 135 mEq/L, along with low osmolality. At the same time, your urine will be inappropriately concentrated with high specific gravity, usually above 1.020. This combination means your body is holding onto water when it should be releasing it. Your doctor will also check that you are normally hydrated and that your kidneys, thyroid, and adrenal glands are working properly.

Rite Aid makes it easy to check your sodium and urine specific gravity through our preventive health panel at Quest Diagnostics locations nationwide. Testing these biomarkers regularly helps catch sodium imbalances early, before symptoms become severe. After diagnosis, your doctor will look for the underlying cause through additional tests like chest X-rays, CT scans, or medication reviews. Monitoring sodium levels over time is essential for managing SIADH and preventing complications.

Treatment options

Restrict fluid intake to 800 to 1,000 mL per day to reduce water retention
Treat the underlying cause, such as stopping trigger medications or treating infections
Use medications called vasopressin receptor antagonists in moderate to severe cases
Add salt tablets or increase dietary sodium in some situations
Give IV saline solution slowly in emergency situations with severe symptoms
Monitor sodium levels frequently through blood tests during treatment
Address nutrition deficiencies that may worsen symptoms
Work with your healthcare team to adjust medications that may be causing SIADH
Get regular follow-up testing to ensure sodium returns to normal range

Frequently asked questions

Lung conditions and lung cancer are the most common causes of SIADH. Pneumonia, tuberculosis, and small cell lung cancer frequently trigger excess ADH production. Certain medications, especially antidepressants and seizure drugs, are also common triggers. Brain injuries and infections can cause SIADH as well.

Sodium can drop well below the normal range of 135 to 145 mEq/L in SIADH. Mild cases may show sodium between 125 and 135 mEq/L. Severe cases can see sodium fall below 115 mEq/L, which is life threatening. The speed of the drop matters as much as the level itself.

SIADH can often be reversed by treating the underlying cause. If a medication is triggering it, stopping that drug may resolve the problem. Treating an infection or managing cancer can also eliminate SIADH. Some cases become chronic and require ongoing fluid restriction and monitoring.

Sodium must be corrected slowly to avoid brain damage. Doctors typically aim to raise sodium by no more than 6 to 8 mEq/L in 24 hours. Correcting sodium too quickly can cause osmotic demyelination syndrome, a serious brain condition. Careful monitoring through frequent blood tests is essential during treatment.

Urine specific gravity above 1.020 in someone with low blood sodium suggests SIADH. This means the urine is too concentrated given the low sodium in the blood. Normally, your body would dilute the urine to get rid of excess water. In SIADH, the kidneys hold onto water inappropriately, creating concentrated urine.

Older adults are at highest risk, especially those taking multiple medications. People with lung disease, lung cancer, or brain injuries face increased risk. Anyone hospitalized for surgery or serious illness can develop temporary SIADH. Those taking certain antidepressants, pain medications, or chemotherapy drugs should watch for symptoms.

Fluid restriction is the main treatment for SIADH, so drinking too much water makes it worse. Most people with SIADH need to limit total fluid intake to 800 to 1,000 mL per day. This includes all beverages and foods with high water content. Your doctor will give you specific limits based on your sodium level.

Testing frequency depends on how severe your SIADH is and whether you are starting treatment. During active treatment, sodium may be checked every 4 to 6 hours. Once stable, testing every few days or weekly helps ensure sodium stays in a safe range. People with chronic SIADH may need monthly monitoring through regular blood tests.

Severe or rapidly developing SIADH can cause brain swelling and permanent damage if untreated. When sodium drops very low or very fast, brain cells swell from excess water. This can lead to seizures, coma, or death in extreme cases. Catching low sodium early through blood testing helps prevent these serious complications.

SIADH and dehydration are opposite problems. Dehydration means too little water in your body, causing high sodium levels. SIADH means too much water retention, causing low sodium levels. In SIADH, your urine is concentrated even though you have too much water in your body. This is the key difference that blood and urine tests reveal.