Testing for Increased Thirst and Urination

By Malcolm Weir, DVM, MSc, MPH; Kristiina Ruotsalo, DVM, DVSc, Dip ACVP & Margo S. Tant BSc, DVM, DVSc

What causes increased thirst and urination?

There are two primary forms of increased thirst and urination. In one, a pet passes large amounts of dilute urine and then drinks excessively to replace the water lost in the urine. In the second form, a pet drinks excessively and then must pass large amounts of dilute urine in to clear the excess water from the body.

Increased thirst and urination are associated with various diseases, and the most common are:

  • Kidney disorders (e.g., kidney failure, kidney infection)
  • Pyometra (uterine infection in intact females)
  • Hormone disorders, including hyperadrenocorticism (overactive adrenal glands—Cushing’s disease), hypoadrenocorticism (adrenal gland failure—Addison’s disease), hyperthyroidism (overactive thyroid gland), diabetes mellitus (sugar diabetes), and diabetes insipidus (see below)
  • High calcium levels in the blood
  • Some types of cancer
  • Liver disease
  • Certain drugs
  • Certain types of electrolyte imbalance
  • Rarely, a behavioral problem called primary polydipsia or psychogenic thirst

How will my veterinarian determine the cause for my pet?

The search for answers begins with a complete history and physical examination. A pet’s history is the information you give the veterinarian about your pet’s illness. History is very important and can provide clues about the cause of increased thirst and urination. For example, a female pet with a history of being in heat six months ago may have increased thirst and urination because of an infected uterus; an elderly cat that is also vomiting might have hyperthyroidism; a pet that is eating well but losing weight may have diabetes mellitus (sugar diabetes).

"The search for answers begins with a complete history and physical examination."

Some drugs can cause increased thirst and urination. Tell your veterinarian about any medication or supplements your pet receives, such as anti-seizure drugs (anticonvulsants), corticosteroids, and diuretics. It helps your veterinarian determine the severity of the problem if you measure how much water your pet drinks in a 24-hour period.

A physical examination involves looking at all parts of the body, listening to the heart and lungs with a stethoscope, and palpating the abdomen (gently squeezing or prodding the abdomen with the fingertips to detect abnormalities of the internal organs). The physical examination may provide clues about the cause of increased thirst and urination. For example, a cat with small rough kidneys may have severe kidney disease; a dog with a sagging abdomen and hair loss might have Cushing’s disease; a dog with enlarged lymph nodes may have a cancer called lymphoma.

History and physical examination are important first steps, but further testing will likely be required, and your veterinarian may recommend screening tests. These simple tests provide information about your pet's overall health and clues about the underlying problem.

What screening tests are recommended?

The most common screening tests are a complete blood count (CBC), a serum biochemistry profile, and a urinalysis. In addition to providing information regarding the possible cause of your pet's symptoms, these screening tests may uncover other conditions that need to be addressed or treated.

Complete blood count (CBC) provides information about the three cell types in the blood: red blood cells, which carry oxygen to the tissues; white blood cells, which fight infection and respond to inflammation; platelets, which help the blood clot. The CBC provides details about the number, size, and shape of the various cell types and identifies the presence of abnormal cells.

In a pet with increased thirst and urination, the CBC may show changes such as:

  • Increased white blood cells may indicate pyometra in an intact female or hyperadrenocorticism. 
  • Increased white blood cells called eosinophils and lymphocytes may indicate hypoadrenocorticism.
  • Abnormal white blood cells may indicate lymphoma (a type of cancer).

Serum biochemistry refers to the chemical analysis of serum, the pale yellow liquid part of blood that remains after the cells and clotting factors are removed. The serum contains many substances, including enzymes, proteins, lipids (fats), glucose (sugar), hormones, electrolytes, and metabolic waste products. Testing for these substances provides information about the health of various organs and tissues in the body, as well as the metabolic state of the animal.

In a pet with increased thirst and urination, the serum biochemistry panel could show some of the following changes:

  • Elevated liver enzymes could indicate liver disease or hyperadrenocorticism.
  • Elevated urea and creatinine are usually a sign of kidney disease.
  • High blood sugar (glucose) level is a sign of diabetes mellitus.
  • Low urea could signal severe liver disease or a condition called medullary washout, which is sometimes seen in pets with longstanding increased thirst and urination. Medullary washout is not serious and is reversible once the increased thirst and urination have improved.
  • Electrolyte abnormalities are consistent with hypoadrenocorticism.

Urinalysis is a simple test that analyses urine's physical and chemical composition. It measures how well the kidneys are working, identifies inflammation and infection in the urinary system, and helps detect diabetes and other metabolic disturbances. Urinalysis is essential for adequately interpreting the serum biochemistry profile and should be done at the same time as blood testing.

In a pet with increased thirst and urination, some of the changes seen on a urinalysis may include:

  • Low urine specific gravity—this means the urine is dilute or watery and confirms that a pet is likely passing increased amounts of urine. If kidney values are elevated simultaneously, kidney disease is likely. If serum kidney values are low, especially urea, severe liver disease, medullary washout, or diabetes insipidus may be the cause.  
  • Glucose—this is a sign of diabetes mellitus.
  • Red blood cells and white blood cells indicate infection and inflammation. They are found with kidney disease, urinary tract infection, and cancer.

What other tests might be required?

Various additional tests might be recommended depending on the results of history, physical exam, and screening tests. Some examples include:

  • Thyroxine (Total T4) in cats. In middle-aged to senior cats, excessively high levels of T4 indicate hyperthyroidism (overactive thyroid gland) is present.
  • Fructosamine. If diabetes mellitus is suspected, a serum fructosamine may be recommended to confirm the diagnosis and establish a starting point to monitor treatment.
  • Radiographs (X-ray) or an ultrasound. These would be recommended to diagnose pyometra.
  • Calcium testing. A high calcium level in the blood can contribute to increased thirst and urination.
  • Adrenal disorder testing. Examples include an adrenocorticotropic hormone (ACTH) stimulation test to diagnose Addison’s disease or a low-dose dexamethasone test (LDDST) to diagnose Cushing’s disease.
  • Liver or kidney failure tests. These may involve function tests, radiographs, an ultrasound, or biopsies. 

If these screening tests are all normal, and your pet continues to pass dilute urine, testing for a disease called diabetes insipidus should be considered.

What is diabetes insipidus?

Diabetes insipidus is a hormonal disorder in which the kidneys do not concentrate urine as they should. Diabetes insipidus is entirely different from diabetes mellitus; the term 'mellitus' refers to the sweetness of the urine in sugar diabetes, and the term 'insipidus' refers to the watery nature of the urine in diabetes insipidus. The hormone involved is called antidiuretic hormone (ADH). This hormone is released from an area within the brain and acts on the kidney to control how much water goes out in the urine. When the body needs water, ADH levels rise, and the kidney holds water back and keeps it from going out in the urine. When excess water is in the body, ADH levels fall, and the kidney allows excess water to flow into the urine.

"Diabetes insipidus is a hormonal disorder in which the kidneys do not concentrate urine as they should."

There are two primary forms of the disease:

  • Central diabetes insipidus (the brain fails to produce proper amounts of ADH)
  • Nephrogenic diabetes insipidus (the kidneys do not respond to ADH even though proper amounts are released from the brain)

How is diabetes insipidus diagnosed?

Three common tests are:

Modified water deprivation test. This measures the kidneys’ ability to concentrate urine if water is withheld from the pet. If a pet can concentrate urine when deprived of water, a diagnosis of primary polydipsia or psychogenic thirst can be made. This is a behavioral problem—affected pets compulsively drink water and drink excessively despite not being thirsty. The kidneys pass large amounts of water in the urine, resulting in dilute urine and increased urination. This is an uncommon disorder.

Vasopressin (ADH) test. This measures the kidneys’ ability to concentrate urine when ADH is administered directly to the pet. If a pet cannot concentrate urine when deprived of water but can concentrate urine when given ADH, a diagnosis of central diabetes insipidus can be made. In this condition, the brain fails to produce proper levels of ADH. Without ADH, the kidney loses large amounts of water in the urine, and the pet must drink excessively to replace the lost water.

Plasma osmolality. This measures how much water is in the blood. Generally, a pet with primary polydipsia/psychogenic thirst will have low plasma osmolality because the blood is ‘diluted’ with all the water the pet is drinking. A pet with diabetes insipidus will have high plasma osmolality (‘thick’ blood) because, without the action of ADH, large amounts of water are lost through the kidneys leaving the body short of water.

Since there can be variability with the plasma osmolality test. It is best used as a screening test rather than the definitive test for diabetes insipidus.

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