Diabetes - Insipidus

Definition of Diabetes Insipidus

Diabetes insipidus is a metabolic disorder that prevents the kidneys from properly reabsorbing normal amounts of water from the fluid that ultimately is excreted as urine. Another name for this is “the inability to properly concentrate urine”. As a result of diabetes insipidus, affected dogs eliminate abnormally large volumes of very dilute urine. They also can develop a number of other symptoms.

Causes of Diabetes Insipidus

Diabetes insipidus can be caused by several different things. In some dogs it is inherited, meaning that the dogs are born with a genetic predisposition to developing the disease. This is called congenital diabetes insipidus. It more commonly is caused by a deficiency in the secretion of antidiuretic hormone (ADH) from the posterior lobe of the pituitary gland (more fully explained below). This can happen as a result of trauma, bacterial or viral infection, pituitary cysts or cancer, all of which can cause damage, degeneration and death of the tissues that are responsible for producing and secreting ADH. This type of diabetes insipidus is referred to as central diabetes insipidus, or CDI.

By way of background, the pituitary gland is located at the base of the brain. It is composed of an anterior and a posterior lobe. The posterior lobe of the pituitary gland, also called the neurohypophysis, acts as a reservoir for various essential hormones, including those that are involved with certain critical aspects of kidney function and regulation of the production and excretion of urine. One of the most important of these is vasopressin, also known as antidiuretic hormone or ADH. The posterior pituitary normally releases ADH into circulation in response to changes in the relative ratio of solids and fluid in an animal’s blood – that is, when circulating blood becomes abnormally high in its solid and low in its fluid components. To a lesser degree, the posterior pituitary gland releases ADH in response to abnormally low blood volume, which can be caused by dehydration, hemorrhage or other causes of blood loss.

Once secreted into the blood stream, ADH travels to the kidneys, where it physically binds to receptors on the surface of specialized kidney cells. This binding normally has the physiological effect of telling the kidneys to increase their reabsorption of water from their collecting ducts and tubules, rather than excreting it in urine. When the system is working properly, this causes an increase in blood volume, a decrease in the ratio of solids to fluids in circulation and production of less dilute, more appropriately concentrated urine. Dogs with central diabetes insipidus do not produce or release enough ADH to signal the kidneys to perform this important fluid-regulation function.

An uncommon form of this disease, called nephrogenic diabetes insipidus (NDI), is caused by renal insensitivity to ADH. NDI occurs when the kidney’s collecting ducts and tubules do not reabsorb water normally, even though plenty of ADH is released from the posterior pituitary lobe, which should trigger the kidneys to pull water out of the urine. The mechanism for NDI is thought to be something that interferes with the ability of circulating ADH to physically bind to its receptors on kidney cells, which prevents the kidneys from receiving the signal to start reabsorbing more water from what otherwise would be excreted as urine. Suggested causes of the decreased binding ability of ADH are endocrine and metabolic disorders such as hyperadrenocorticism (Cushing’s disease), high blood levels of calcium (hypercalcemia), high circulating levels of the hormone aldosterone or low blood levels of potassium (hypokalemia), and circulation of toxins produced by the bacterial microorganism, Escherichia coli (E.coli). Primary kidney disease or infection may also contribute to NDI. The precise connection between these disorders and the development of nephrogenic diabetes insipidus is not well-described or well-understood in medical circles.

Psychogenic diabetes insipidus – more appropriately called psychogenic polydipsia – is a symptomatically related condition that also involves the excretion of large volumes of dilute urine (called polyuria). It results from a neurological disorder of thirst control or as a result of some behavioral problem that triggers excessive water intake. In either case, the abnormally large volume of excreted urine in dogs with psychogenic polydipsia is caused by excessive water intake (polydipsia), rather than by some primary problem with kidney, pituitary or hypothalamic function.

The cause of a dog’s diabetes insipidus may never be determined. This is called idiopathic diabetes insipidus. “Idiopathic” means something that occurs without a known cause.

Prevention of Diabetes Insipidus

There is no one way to prevent dogs from developing diabetes insipidus, because its causes can be so varied. However, it is wise to avoid situations or circumstances that might markedly increase a dog’s water loss or water intake.

Special Notes

Diabetes insipidus of any form is fairly rare in domestic dogs. It is much less common than diabetes mellitus, which involves defects in the production of or response to insulin, rather than problems with the kidneys’ ability to reabsorb water.