SIADH and diabetes insipidus, or arginine vasopressin disorder (AVD), involve opposite effects on the same hormone. AVD causes excessive urination and thirst, while SIADH leads to increased water retention.

Syndrome of inappropriate antidiuretic hormone secretion (SIADH) and AVD — which healthcare professionals previously called diabetes insipidus — are both conditions that affect fluid balance and electrolyte levels.

In SIADH, a person’s body does not remove enough fluids. The pituitary gland in the brain and other sources release too much vasopressin, a hormone that helps the body retain fluids.

AVD causes a person to pass too much urine. This often occurs because the pituitary gland produces too little vasopressin or because the receptors in the kidneys do not respond enough to vasopressin.

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SIADH and AVD affect vasopressin secretion in opposite ways. To understand the differences, it may be helpful to think of vasopressin regulation as a spectrum, with SIADH at one end and AVD at the other.

People with AVD pass too much urine — sometimes up to 20 quarts per day, while a person without AVD typically passes 1 to 3 quarts per day. Conversely, people with SIADH retain too much fluid, which causes a low sodium concentration in their blood, or hyponatremia.

The conditions cause various symptoms and have a range of causes, many of which differ from one another. This article explains these below.

SIADH and AVD both affect water and electrolyte balance. Electrolytes, including sodium, are vital substances in the blood that drive chemical reactions throughout the body. Vasopressin helps regulate fluid levels and keep electrolyte levels balanced and regular.

The two conditions also share some causes. For example, cancers may contribute to either condition. And people with either condition may experience symptoms of electrolyte imbalance, including:

  • fatigue
  • sleepiness
  • muscle weakness
  • seizures

Many of the causes and symptoms of SIADH differ from those of AVD.

Causes of SIADH

SIADH is most often a result of another health condition. Conditions that may lead to SIADH include:

  • central nervous system problems
  • cancers, including small cell lung cancer, head and neck cancers, olfactory neuroblastoma, and extrapulmonary small cell carcinoma
  • side effects of medications — most commonly carbamazepine, oxcarbazepine, chlorpropamide, cyclophosphamide, and selective serotonin reuptake inhibitors (SSRIs)
  • surgery
  • lung diseases, especially pneumonia
  • hormone imbalances such as hypopituitarism or hypothyroidism
  • hormone supplementation, such as with vasopressin, desmopressin, or oxytocin
  • HIV
  • hereditary SIADH, an inherited condition that constantly activates the receptors in the kidneys that interact with vasopressin

Symptoms of SIADH

The severity of low sodium levels in the blood usually dictates which SIADH symptoms a person develops. The earliest symptoms include nausea, weakness, and vomiting. Once hyponatremia becomes severe, it may cause other symptoms, including:

  • lethargy and drowsiness
  • headaches
  • reduced responsiveness to the environment
  • seizures

AVD has several types, which may result from a range of underlying causes and have different signs and symptoms.

Causes of AVD

The cause of AVD depends on which of the following types a person has:

Arginine vasopressin deficiency (AVP-D)

Healthcare professionals previously referred to this type as central diabetes insipidus. It occurs when the pituitary gland does not release enough vasopressin.

AVP-D leads to the kidneys expelling fluids in urine rather than the cells retaining fluids. This condition can result from hypothalamus or pituitary damage after surgery, inflammation, an autoimmune condition, or head trauma.

AVP resistance (AVP-R)

Healthcare professionals previously referred to this type as nephrogenic diabetes insipidus. It occurs when the kidney receptors that interact with vasopressin do not respond as they should.

Certain medications, low potassium levels, high calcium levels, and urinary tract blockages can cause this type. People can also inherit AVP-R from their parents. In rare cases, chronic kidney disease can lead to AVP-R.

Primary polydipsia

Also known as dipsogenic diabetes insipidus, this type results from a problem with the hypothalamus. It causes extreme, ongoing thirst, which leads a person to drink more fluids.

Causes of this type include hypothalamus damage from infection, inflammation, tumors, head injury, or surgery, as well as some medications and mental health problems.

Gestational form of AVD

Healthcare professionals previously referred to this as gestational diabetes insipidus. It is a rare and temporary subtype that occurs during pregnancy.

The placenta may make too much of an enzyme that breaks down vasopressin. Pregnancy with multiples, such as twins or triplets, makes this more likely. Diseases that affect liver function may also increase the risk.

Symptoms of AVD

Symptoms of AVD include:

People with AVP-D might also experience:

  • weakness
  • sleepiness
  • fatigue
  • muscle pain

Treating both conditions involves restoring balance to vasopressin levels. Medications can help regulate vasopressin. People may also need other treatments to manage underlying health conditions such as cancer, inflammation, or head trauma.

Desmopressin is a lab-made form of vasopressin that people with AVD can take to boost their levels. People can take it by mouth or nose, via injections, or through an intravenous (IV) infusion. Gestational AVD generally resolves without treatment.

People with SIADH may need to consume less than 800 milliliters of water per day and receive sodium supplements, either in pill form or as an IV injection. Vasopressin receptor antagonists can help block vasopressin’s water-retaining effect.

No research has yet documented a person having both SIADH and AVD at the same time.

It is possible for a person to have one condition after the other, but this is extremely rare. For example, a 2022 case study describes a person who developed AVP-D immediately after SIADH as an effect of a Rathke’s cleft cyst.

SIADH and AVD sit at opposite ends of the vasopressin regulation spectrum. People with SIADH produce too much vasopressin, which leads to excessive water retention. Those with AVD produce too little vasopressin, or the vasopressin receptors in their kidneys do not respond as they should. This leads to more urination than usual.

The two conditions can affect electrolyte and water levels in different ways. They can lead to similar symptoms of electrolyte imbalance, even though they have different effects on fluid and sodium levels and a range of causes. The two conditions may also have some similar causes, such as head trauma and certain cancers.