Syndrome of inappropriate antidiuretic hormone secretion (SIADH) is a condition in which there are high amounts of antidiuretic hormone (ADH) released from the pituitary gland or other sources. Show
Image Credit: Yeexin Richelle/Shutterstock.com SIADH often leads to low levels of sodium in the blood (hyponatremia), high urine osmolality and excessive sodium in the urine, and low serum osmolality. It is the most common cause of hyponatremia. There are four types of SIADH: type A, B, C, and D. Type A occurs independently of plasma osmolality. There are high levels of ADH and urine osmolality. Type B is characterized by a constant release of ADH. Type C is characterized by a baseline plasma sodium concentration that is stable, but lower than normal. Type D shows normal ADH levels but high urine osmolality. The Role of Antidiuretic Hormone in the BodyAntidiuretic hormone is also called arginine vasopressin (AVP). It is produced in the hypothalamus in the brain. ADH has been found to help to regulate the osmotic balance in the body, which refers to the amounts of electrolytes and non-electrolytes in the cells, tissues, and interstitial fluid. It also regulates sodium levels, blood pressure, and helps the kidneys to function normally. ADH is also responsible for regulating water retention in the body. The more ADH there is the body, the more water will be retained. Conversely, less water will be retained if ADH levels are low. Mechanism of SIADHThe mechanism by which SIADH develops primarily involves the kidneys. Inside the kidneys are tubes called nephrons that move fluid and waste along to the bladder and control how much water leaves the body. Fluids and electrolytes can pass through the nephrons to be released into the blood if necessary. ADH affects two particular areas of the nephrons, called the distal convoluted tubules and the collecting ducts. These tubes reabsorb water back into the blood and are lined with proteins called aquaporins that allow water to move in and out of the cells that make up the distal convoluted tubule and the collecting ducts. When there are low ADH levels, less aquaporins are lining the walls of the nephrons, meaning that the water passes through the collecting ducts and distal convoluted tubules and leaves the body as dilute urine. High ADH levels mean that the aquaporins release the water back into the blood. When water is released back into the blood, plasma osmolality drops, leading to the blood becoming diluted. This causes the pituitary gland to lower the amount of ADH being produced. As a result, there is a high amount of dilute urine produced, which brings the osmolality levels back to normal. However, if ADH is still produced when the plasma osmolality is low, more water is retained and it continues to dilute the blood and the electrolytes in it, such as sodium. Dilute blood also takes up more space in the blood vessels, which causes a hormone called aldosterone to get rid of sodium through the urine when there are already low levels of sodium. This leads to symptoms of SIADH, which occur because of the low levels of sodium in the blood. Causes of SIADHUsually, SIADH occurs because of another disease. However, there is a range of causes of SIADH. Central nervous system (CNS) problemsCNS disorders such as stroke, infections, trauma, hemorrhage, and psychosis all increase the amount of ADH released by the pituitary gland. CancerTumors in the lung, in particular small cell lung cancer (SCLC), may increase the ADH levels in some cases. Other cancers such as pancreatic cancer, duodenum cancer, and head and neck cancers can also produce ADH. Some cancer treatments can also cause SIADH. DrugsCertain types of drugs may cause SIADH. These drugs may include:
SurgeryAbdominal and thoracic surgery can increase ADH, as well as surgery to the pituitary gland. Lung diseaseLung disorders such as pneumonia, bronchial asthma, atelectasis, acute respiratory failure, and pneumothorax can cause SIADH, although the mechanisms through which this happens is not known. Image Credit: create jobs 51/Shutterstock.com Hormone administrationA person may develop SIADH after being given certain hormones, such as vasopressin (which is simply ADH in drug form), desmopressin, or oxytocin. All of these drugs increase the activity of vasopressor-2 (V2) receptors in the kidneys. Human Immunodeficiency Virus (HIV)HIV may cause a person to be more susceptible to infections and malignant diseases, which can cause hyponatremia and SIADH. Acquired immune deficiency syndrome (AIDs) is also associated with SIADH. Hereditary SIADHGenetic syndromes such as nephrogenic syndrome and hypothalamic syndrome have been associated with SIADH. In nephrogenic syndrome, a gene mutation in the V2 receptor on the X chromosome, which is responsible for the reabsorption of water in the collecting ducts, is linked to SIADH. In hypothalamic syndrome, a mutation in the transient receptor potential vanilloid type 4 (TRPV4) has been linked to SIADH. The body’s ability to sense hypo-osmolality is impaired with hypothalamic syndrome, so ADH production may not be slowed. Symptoms of SIADHThe symptoms of SIADH are caused by the loss of sodium in the blood.
Diagnosis of SIADHA set of criteria was created in 1967 by William Schwartz and Frederic Bartter for the diagnosis of SIADH that are still in use today. These criteria are:
Treatment for SIADHThe type of treatment a person receives depends on the severity of their symptoms, along with their overall health, age, and any other existing conditions they may have. Those with mild to moderate symptoms will be asked to restrict their oral water intake to under 800 milliliters a day. For chronic hyponatremia, oral salt tablets can help to increase the sodium levels in the body, or intravenous saline can be administered. Loop diuretics used with salt tablets can help to decrease urine concentration and increase water excretion. If a person has chronic SIADH, they may need to restrict their fluid and water intake permanently. Medication can be used to treat SIADH, including medications that reduce the production of ADH, and medications that regulate body fluid volume. Surgical options include removing tumors that may be producing ADH. SummarySyndrome of inappropriate antidiuretic hormone secretion causes low sodium levels in the blood through excessive water retention. The extra water diluting the blood lowers the amounts of certain electrolytes in the blood and can cause serious symptoms such as seizures, coma, and psychological symptoms such as mood changes and hallucinations. Sodium levels can be brought back to their normal rates by restricting the amount of water or fluid a person takes in, which may be a permanent solution if sodium levels are chronically low. Although SIADH can be inherited, it is usually caused by an underlying disease, which should be treated along with SIADH. Sources
Further Reading
What causes syndrome of inappropriate antidiuretic hormone secretion?SIADH tends to occur in people with heart failure or people with a diseased hypothalamus (the part of the brain that works directly with the pituitary gland to produce hormones). In other cases, a certain cancer (elsewhere in the body) may produce the antidiuretic hormone, especially certain lung cancers.
What is the most serious complication of SIADH?A low sodium level or hyponatremia is a major complication of SIADH and is responsible for many of the symptoms of SIADH. Early symptoms may be mild and include cramping, nausea, and vomiting. In severe cases, SIADH can cause confusion, seizures, and coma.
What is the most common disease related to antidiuretic hormone?Diabetes insipidus is caused by a lack of antidiuretic hormone (ADH), also called vasopressin, which prevents dehydration, or the kidney's inability to respond to ADH. ADH enables the kidneys to retain water in the body. The hormone is produced in a region of the brain called the hypothalamus.
What drugs can cause SIADH?Five drug classes (antidepressants, anticonvulsants, antipsychotic agents, cytotoxic agents and pain medications) were implicated in 82.3% of patients diagnosed with drug‐associated SIADH. Specific serotonin reuptake inhibitors and carbamazepine were commonly implicated.
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