Diabetes insipidus is a hormonal disorder in which the kidneys excrete excessive amounts of water. This results in the production of large volumes of very watery urine, often several liters a day. The loss of water causes intense thirst, leading affected animals to drink extremely large amounts of water.
To better understand the condition, here is a simplified explanation of how water excretion is regulated in healthy animals. The antidiuretic hormone (ADH), also known as vasopressin, is primarily responsible for this regulation. It is released from the pituitary gland when:
ADH binds to receptors in the blood vessels and in the kidneys. This causes the blood vessels to constrict and reduces water excretion by the kidneys. As a result, blood pressure and blood volume return to their normal levels.
There are two types of diabetes insipidus:
Central diabetes insipidus results from reduced production and/or secretion of ADH in the brain. This can arise due to a disorder in the hypothalamus (a part of the diencephalon where ADH is produced), in the neurohypophysis (a part of the pituitary gland where ADH is stored), or in the nerve cells between them. Possible causes include:
In nephrogenic diabetes insipidus, the ADH receptors in the kidney do not respond, or do not respond sufficiently, to ADH. As a result, the kidney excretes too much water despite normal ADH levels. In rare cases, this may be a congenital defect, but it is usually triggered by the following underlying conditions or factors:
The typical symptoms of diabetes insipidus are marked, often sudden-onset polyuria (increased urine output) and polydipsia (increased water intake). Because affected animals often eat less due to excessive thirst, this can lead to weight loss. If central diabetes insipidus is caused by head injuries, tumors, or inflammation, additional neurological symptoms may occur. In nephrogenic diabetes insipidus, symptoms of the underlying disease are usually present. If there is no free access to drinking water, dehydration occurs, which can reach life-threatening levels. However, with sufficient water available, diabetes insipidus often remains undiagnosed for a long time, as the increased urine output is balanced by increased water intake.
If you notice polyuria and polydipsia in your pet, you should measure the amount of water consumed to ensure that it is indeed polyuria/polydipsia and not dysuria (=difficult, painful urination), as in the case of a bladder infection, or incontinence (=involuntary loss of urine, see relevant article).
The diagnosis is made by systematically ruling out possible causes.
First, based on your report of polyuria/polydipsia, your veterinarian will perform a general examination as well as blood and urine tests. This helps identify nephrogenic diabetes insipidus caused by underlying conditions. In addition, your veterinarian will look for signs of other conditions that are much more common causes of polyuria/polydipsia than diabetes insipidus. In particular, diabetes mellitus (see related article) must be ruled out. Depending on the underlying condition, further tests or X-ray and ultrasound examinations may be necessary.
If your veterinarian has not found a cause for the polyuria/polydipsia with these examinations, central diabetes insipidus, congenital nephrogenic diabetes insipidus, and psychogenic polydipsia (=compulsive, excessive drinking as part of a mental illness) remain as possible causes.
To test for central diabetes insipidus, a trial treatment with a synthetic ADH analog is performed. This is a medication that has the same effect as the ADH produced by the brain. It can be administered at home. Subsequently, fluid intake is measured and urine is examined daily. If central diabetes insipidus is present, there is a significant decrease in fluid intake and an increase in specific gravity of the urine (=a measure of urine concentration) within two days. In contrast, no or only minimal changes are observed in cases of nephrogenic diabetes insipidus and psychogenic polydipsia. However, since fluid intake may also decrease slightly in other conditions, the therapeutic trial should be interrupted for five to seven days after five to seven days and then repeated for another five to seven days. Only if fluid intake is significantly reduced during the repeat trial can a diagnosis of central diabetes insipidus be made. A measurement of ADH alone is not conclusive, as ADH secretion occurs rhythmically, which is why measured values cannot be interpreted with certainty.
If no cause has yet been found, the distinction between congenital nephrogenic diabetes insipidus and psychogenic polydipsia is made via a thirst test. However, due to the risk of dehydration, this test may only be performed under close supervision at a veterinary practice or clinic. It begins with a three-day gradual reduction in the amount of drinking water. Afterward, food and water are completely removed, and the bladder is emptied via a urinary catheter. The actual test lasts about four to eight hours. At the start and every one to two hours, the animal is clinically examined, weighed, the bladder is emptied, and urine and blood tests are performed. If the specific gravity of the urine increases during the thirst period, this indicates psychogenic polydipsia. In nephrogenic diabetes insipidus, there is no increase or only a very slight increase in the specific gravity of the urine.
In cases of central diabetes insipidus, a synthetic ADH analogue is administered on a long-term basis. Your pet should also have access to drinking water at all times. In cases of nephrogenic diabetes insipidus, the underlying condition is treated.
In cases of idiopathic central diabetes insipidus, the prognosis is good with long-term treatment using a synthetic ADH analogue. It is essential to ensure constant access to water. For all other forms, the prognosis depends on the course of the underlying disease.
Due to the wide variety of underlying conditions and contributing factors, there is no specific preventive measure that can be taken. However, an annual checkup, including blood and urine tests, can help detect and treat conditions early on. If you notice polyuria or polydipsia in your pet, take it to your veterinarian as soon as possible.
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