An antidiuretic is a substance that helps the body conserve water by reducing the amount of urine produced. When the body loses too much water, these substances signal the kidneys to reabsorb more fluid, preventing dehydration. This process helps maintain proper hydration and fluid balance, keeping the body’s internal environment stable.
The Body’s Natural Antidiuretic
The human body produces its own natural antidiuretic, a hormone known as antidiuretic hormone (ADH), which is also called vasopressin. This hormone is synthesized by nerve cells in the hypothalamus and transported to the posterior pituitary gland, where it is stored and released into the bloodstream.
Once released, ADH travels to its primary target: the kidneys. It acts on the final parts of the kidney tubules and the collecting ducts, increasing their permeability to water. This is achieved by stimulating the insertion of special water channels, called aquaporin-2 channels, into the cell membranes of the tubule walls.
This increased permeability allows water that would otherwise be excreted as urine to be reabsorbed back into the bloodstream. As more water returns to the body, the remaining urine becomes more concentrated, and water loss is reduced. Higher concentrations of ADH can also cause blood vessels to narrow, which increases blood pressure.
Regulation of Antidiuretic Hormone
The release of ADH is tightly regulated to maintain the body’s fluid balance. The primary factor controlling its secretion is the osmotic pressure of the blood, which is the concentration of solutes like salt. Specialized nerve cells in the hypothalamus, called osmoreceptors, constantly monitor this concentration.
When the body becomes dehydrated from low water intake or high salt consumption, the concentration of solutes in the blood rises. The osmoreceptors detect this and signal the posterior pituitary gland to release ADH. Other triggers for ADH release include a significant drop in blood volume or blood pressure, pain, and nausea.
Conversely, when a person drinks large quantities of fluid, the blood becomes more dilute, and its osmotic pressure decreases. The osmoreceptors detect this change and inhibit the release of ADH. With less ADH acting on the kidneys, less water is reabsorbed, and the body produces a larger volume of dilute urine. Alcohol is a well-known substance that suppresses ADH, leading to increased urination and potential dehydration.
Medical Uses of Antidiuretic Drugs
Synthetic versions of ADH have been developed for medical use, with the most common being a drug called desmopressin. Desmopressin is a modified form of the natural hormone, designed to have a more potent and longer-lasting antidiuretic effect while minimizing its impact on blood pressure. It is available in various forms, including tablets, nasal sprays, and injections.
One of the primary uses for desmopressin is the treatment of central diabetes insipidus, a condition where the body does not produce enough ADH. By replacing the missing hormone, desmopressin helps the kidneys reabsorb water, thereby controlling the extreme thirst and excessive urination that characterize the disorder.
Desmopressin is also widely used to manage nocturnal enuresis, or bedwetting, in children and adults. Taken at bedtime, the medication reduces urine production overnight. A secondary application for desmopressin is in the management of certain mild bleeding disorders, such as von Willebrand disease, where it works by stimulating the release of clotting factors.
Conditions of Hormone Imbalance
Imbalances in the production or function of ADH can lead to medical conditions affecting the body’s fluid and electrolyte balance. These conditions fall into two categories: having too little or too much of the hormone.
ADH Deficiency
When the body does not produce enough ADH due to damage to the hypothalamus or pituitary gland, a condition known as central diabetes insipidus occurs. This is not related to the more common diabetes mellitus, which involves blood sugar. The lack of ADH means the kidneys are unable to conserve water, leading to the excretion of massive volumes of very dilute urine, a symptom called polyuria. People with this condition can urinate between 3 and 20 quarts a day.
This water loss triggers an intense thirst, known as polydipsia, as the body attempts to compensate for the dehydration. Patients feel compelled to drink large amounts of fluids to keep up with their fluid losses. If fluid intake cannot match the urinary output, severe dehydration and high blood sodium levels can develop, leading to confusion, seizures, or other neurological symptoms.
ADH Excess
The opposite problem occurs in the Syndrome of Inappropriate Antidiuretic Hormone secretion (SIADH), where the body produces too much ADH. This excess hormone can be secreted by the pituitary gland or come from an ectopic source, such as certain tumors. The high levels of ADH cause the kidneys to retain excessive amounts of water, even when the body is already well-hydrated.
This water retention leads to a dilution of the blood, causing a condition called hyponatremia, where sodium levels become dangerously low. Symptoms of SIADH are related to this low sodium and can vary depending on how quickly the level drops. Mild cases may cause nausea, headache, or muscle cramps, while more severe cases can lead to confusion, seizures, and coma as the brain cells swell with excess water.