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Syndrome of inappropriate antidiuretic hormone

Syndrome of inappropriate antidiuretic hormone (SIADH) is a group of symptoms that develop when there is too much antidiuretic hormone (ADH) in the body. ADH plays an important role in regulating the balance of fluids in the body. It lowers the amount of urine the body makes and increases the amount of water the kidneys take up. Too much ADH leads to water retention, electrolyte imbalances and a low level of sodium in the blood (called hyponatremia or water intoxication).

SIADH that develops suddenly can be life-threatening.

Causes

The following cancers can cause SIADH:

  • small cell lung cancer
  • non–small cell lung cancer
  • mesothelioma
  • lymphoma
  • Ewing sarcoma
  • thymoma, which is a type of thymus cancer
  • primary brain tumours
  • head and neck cancers
  • stomach cancer
  • duodenum cancer
  • pancreatic cancer
  • bladder cancer
  • prostate cancer
  • uterine cancer

Some cancer treatments can also lead to SIADH. These include chemotherapy drugs such as cyclophosphamide (Cytoxan, Procytox), ifosphamide (Ifex) and vincristine (Oncovin). Other medicines used in cancer treatment, including opiate pain medicines such as morphine, can also lead to SIADH.

Symptoms

Symptoms of SIADH can vary depending on how low the sodium level in the blood is. Symptoms may be mild and vague at first. They can get worse as the sodium level goes down.

Symptoms of SIADH include:

  • fatigue
  • anorexia
  • headache
  • nausea
  • vomiting
  • muscle cramps
  • worsening mental status, including lethargy, weakness, irritability, agitation, confusion, combativeness, delirium, disorientation, hallucinations and poor balance
  • seizures
  • coma

Report symptoms to your doctor or healthcare team as soon as possible.

Diagnosis

Your doctor will try to find the cause of SIADH. This usually includes physical and neurological exams. Your doctor will ask questions about your symptoms and health history, and check your coordination and how well your muscles and reflexes are working. You may also need to have the following tests.

Blood chemistry tests are done to check the levels of sodium in the blood, blood urea nitrogen (BUN), creatinine, albumin and uric acid.

Urine tests are done to check the level of sodium in the urine. The amount of sodium in the urine will be higher if you have SIADH.

Osmolality is a test that measures the concentration of all the chemicals in the fluid part of the blood or in urine. Osmolality tests are used to check the body’s water and electrolyte balance. With SIADH, blood osmolality will be lowered and urine osmolality will be greater than blood osmolality.

Imaging tests, including chest x-ray and CT scan may be done to see if there is evidence of tumours.

Find out more about these tests and procedures.

Treating SIADH

Once SIADH is diagnosed, your healthcare team will treat it. They will closely monitor your sodium levels, kidney function and neurological symptoms. They will also suggest measures to increase the level of sodium in the blood. Managing hyponatremia depends on how severe it is.

For mild to moderate hyponatremia, you may only need to limit how much you drink until the sodium level returns to normal. You may also be given medicines to increase sodium levels, such as tolvaptan (SAMSCA), lithium and urea.

For severe hyponatremia, you may be given saline, or salt water, solution through a needle in a vein (intravenous infusion) to increase sodium levels. You may also need to be given medicine to increase your urine output.

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