Testing for Increased Thirst and Urination

What are the causes of increased thirst and urination?

There are two basic forms of increased thirst and urination. In one, a pet passes large amounts of dilute (pale watery) urine 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 order to clear the excess water from the body.

Increased thirst and urination are associated with a variety of diseases. The following list includes some of the more common disorders:

• kidney disorders such as kidney failure and kidney infection
• infection of the uterus in intact females called pyometra
• hormone disorders including hyperadrenocorticism (over active 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 levels of calcium 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

This list is huge! How can we possibly determine the cause in 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 important 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 hyperthyroid disease; a pet that is eating well but losing weight may have diabetes mellitus (sugar diabetes).

"...you should tell your veterinarian about any medication or supplements your pet is receiving…"

Some drugs can cause increased thirst and urination and you should tell your veterinarian about any medication or supplements your pet is receiving such as anti-seizure drugs (“anticonvulsants”), corticosteroids, and diuretics (“water pills”). It would also help if you measured exactly how much water your pet is drinking in a 24 hour period; this will help your veterinarian determine the severity of the problem.

Physical examination involves looking at all parts of the body, and typically includes 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). Physical examination may provide clues about the cause of increased thirst and urination. For example, a cat with small rough kidneys may have serious 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 are simple tests that provide information about the overall health of your pet and often provide 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.

What can these screening tests tell us?

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.

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

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

• increased number of white blood cells, which could indicate pyometra in an intact female, or hyperadrenocorticism (Cushing’s disease).
• increased number of certain types of white blood cells (eosinophils and lymphocytes), which could indicate hypoadrenocorticism (Addison’s disease).
• abnormal white blood cells which may be an indicator of a type of cancer called lymphoma

(b) Serum biochemistry refers to the chemical analysis of serum, which is the pale yellow liquid part of the blood that remains after the cells and clotting factors have been removed. 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. (See article Serum Biochemistry)

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

• elevated levels of liver enzymes, which could indicate liver disease or hyperadrenocorticism (Cushing’s Disease)
• elevated levels of urea and creatinine, which usually are a sign of kidney disease
• high blood sugar (glucose) level which is a sign of diabetes mellitus
• low urea, which could signal severe liver disease or a condition call medullary washout sometimes seen in pets with longstanding increased thirst and urination. Medullary wash out is not serious and is reversible once the increased thirst and urination has improved.
• electrolyte abnormalities consistent with hypoadrenocorticism (Addison’s disease)

(c) Urinalysis is a simple test that analyses the physical and chemical composition of urine. It measures how well the kidneys are working, identifies inflammation and infection in the urinary system, and helps to detect diabetes and other metabolic disturbances. Urinalysis is important for the proper interpretation of the serum biochemistry profile and should be done at the same time as blood testing. (See article Urinalysis).

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

• low urine specific gravity. This means the urine is dilute or watery and confirms a pet is likely passing increased amounts of urine. If kidney values are elevated at the same time, then kidney disease likely present. If serum kidney values are low, especially urea, then severe liver disease, medullary washout, or diabetes insipidus may be the cause.  
• glucose (sugar). This is a sign of diabetes mellitus.
• red blood cells and white blood cells. These indicate infection and inflammation. They are found with kidney disease, urinary tract infection, and cancer.

What other tests might be required?

A variety of 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 older cats, excessively high levels of T4 indicate hyperthyroidism (over active 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.
• X-rays, ultrasound – These would be recommended to diagnose pyometra in an intact female pet.  
• calcium testing – a high calcium level in the blood can contribute to increased thirst and urination.
• testing for adrenal disorders. For example, ACTH stimulation test to diagnose Addison’s disease, or Low Dose Dexamethasone test to diagnose Cushing’s disease.
• testing to assess liver or kidney failure. These may involve function tests, X-rays, ultrasound, or biopsies. 

What if the cause of increased thirst and urination is not found?

If the screening tests are all normal and the pet continues to pass dilute urine, then testing for a disease call 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 completely different than diabetes mellitus (the term 'mellitus' refers to the sweetness of the urine in sugar diabetes, while 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 there is excess water in the body, ADH levels fall and the kidney allows excess water to flow into the urine.

There are two main forms of the disease:

• Central Diabetes Insipidus - in which the brain fails to produce proper amounts of ADH,
• Nephrogenic Diabetes Insipidus - in which the kidneys do not respond to ADH, even though proper amounts are released from the brain. 

What is the test for Diabetes Insipidus?

There are several tests to diagnose diabetes insipidus. Three common ones are:

• Modified water deprivation test – which measures the kidneys’ ability to concentrate urine if water is withheld from the pet.
• Vasopressin (ADH) test – which measures the kidneys’ ability to concentrate urine when anti-diuretic hormone (ADH) is administered directly to the pet.
• Plasma osmolality – which measures how much water is in the blood.  

How are the results of these tests interpreted?

In general, the tests can be interpreted as follows:

(a) Modified Water Deprivation test:

• If a pet is able to concentrate urine when deprived of water, then a diagnosis of primary polydipsia or psychogenic thirst can be made. This is a behavioural problem. Affected pets have a compulsion to drink water and will drink excessively even though they are not thirsty. The kidneys pass large amounts of water in the urine resulting in dilute urine and increased urination. This is an uncommon disorder.

(b) Vasopressin test:

• If a pet is unable to concentrate urine when deprived of water but can concentrate urine when given ADH, then a diagnosis of central diabetes insipidus can be made. In this condition the brain fails to produce proper levels of ADH hormone. Without ADH the kidney loses large amounts of water in the urine and the pet must drink excessively to replace the lost water.
• If a pet is unable to concentrate urine either when water is withheld or when ADH is given, then a diagnosis of nephrogenic diabetes insipidus is most likely. In this condition the brain produces proper levels of hormone but the kidney is unable to respond. As a result large amounts of water are lost in the urine and the pet must drink excessively to replace lost water.

Plasma Osmolality:

• In general, a pet with primary polydipsia/psychogenic thirst will have low plasma osmolality because the blood is “diluted” by all the water the pet is drinking.
•  A pet with diabetes insipidus will have high plasma osmolality (think of it as “thick” blood), because without the action of ADH, large amounts of water are lost through the kidneys leaving the body short of water.

It is important to note that there can be quite a bit of variability with the plasma osmolality test. It is best used as a screening test, rather than the definitive test for diabetes insipidus.