Testing for Weakness

By Kristiina Ruotsalo, DVM, DVSc, Dip ACVP & Margo S. Tant BSc, DVM, DVSc

How can I tell if my pet is feeling weak?

Pets that feel weak often have difficulty getting to their feet and move slowly or unsteadily. Other signs include shaky muscles, fainting, or collapse. You may find your pet does not want to exercise, seems dull, and does not respond when you call. Weakness may be constant or may be intermittent (comes and goes). Intermittent weakness is sometimes called “episodic weakness” because your pet may show only episodes of weakness and is fine otherwise.

What might be causing my pet’s weakness?

"Weakness can be caused by many different problems and can involve a variety of body systems."

Weakness can be caused by many different problems and can involve a variety of body systems. Some common causes of weakness include:

  • Heart disease – heart failure, poor circulation, irregular heart beat

  • Anemia – “thin” or “iron-poor” blood due to many causes

  • Metabolic disease – low blood sugar (hypoglycemia), low calcium (hypocalcemia), high potassium (hyperkalemia), low potassium (hypokalemia)

  • Hormonal disease – hypoparathyroidism (failure of the parathyroid gland), hypothyroidism (underactive thyroid gland in dogs), hyperthyroidism (overactive thyroid gland in cats), hypoadrenocorticism (adrenal gland failure called “Addison’s Disease” )

  • Bone/Muscle disorders – arthritis, joint problems

  • Nerve/Muscle disorders – spinal cord disease such as collapsed disc, nerve damage, myasthenia gravis (rare), seizures

  • Cancer


This list is huge. How do we determine the cause of my pet’s weakness?

Finding the cause of your pet’s weakness starts with a complete history and physical examination. Your pet’s history is the information you give the veterinarian about your pet’s illness. In a pet showing weakness, this would include details about how long the signs have been present and whether the weakness is continuous or intermittent. You would want to report any changes you have noticed in your pet’s activity, attitude, appetite, water intake, bowel habits, and urination. It would also be important to tell your veterinarian if your pet is taking medications such as insulin, or if you have seen your pet eat anything unusual.

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 identify abnormalities inside the body). A complete physical examination may give clues about the cause of weakness. For example, abnormal heart sounds could signal underlying heart disease; very pale gums could be a sign of bleeding or serious organ failure; the presence of an abdominal mass could signal a tumor; and hair loss could be a sign of hypothyroidism in a dog.

History and physical examination are very important, but further testing will likely be necessary and your veterinarian may recommend doing screening tests. These are a series of simple tests that provide information about the overall health of your pet and give clues about the underlying problem. The most commonly recommended screening tests for weakness include: complete blood count (CBC), serum biochemistry profile, urinalysis, and in a dog total thyroxine (T4). Based on the results of these screening tests, additional specific tests may be recommended.


What can these screening tests tell us?

(a) Complete Blood Count (CBC): This is a simple blood test that provides information about the different cell types in blood. These include 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 cells types, and identifies the presence of abnormal cells in circulation.

In a pet with signs of weakness, changes seen on a CBC could include:

  • Anemia - this means the number of red blood cells and the amount of hemoglobin in the blood are lower than normal. Anemia is a common cause of weakness and is seen with a variety of disorders including bleeding, longstanding disease, hypothyroidism (dogs), parasitism, poor nutrition, or cancer.

  • High numbers of white blood cells with an increased proportion of immature white blood cells - this is usually a sign of severe underlying infection and could explain weakness.

  • Unusual pattern of white blood cells - the presence of high numbers of both lymphocytes and eosinophils (types of white blood cells) could be a sign of Addison’s disease (adrenal gland failure), which is associated with episodic weakness.

  • Abnormal cells - this is often a sign of bone marrow disease or bone marrow cancer (leukemia).


(b) Serum biochemistry refers to the chemical analysis of serum, which is the pale yellow liquid part of 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.

In a pet with signs of weakness, a serum biochemistry profile might show changes such as:

  • elevation in liver or kidney values –  could indicate organ damage or failure

  • low glucose – an important cause of weakness that could be related to liver disease, some types of cancer, severe infections, and over medication with insulin

  • low calcium – an important cause of weakness that can be related to dietary or hormonal problems

  • altered electrolytes – electrolytes are salts and minerals found in the blood. Changes in electrolytes, especially sodium and potassium, are often seen with Addison's disease and kidney disease.

(c) Urinalysis is a routine test that analyzes 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.

In a weak pet, the urinalysis might show changes such as:

  • low urine specific gravity (pale watery urine) – if the kidney values are high on serum biochemistry then kidney disease is present.

  • large numbers of red blood cells or white blood cells – this usually indicates inflammation or infection in the urinary system and can be associated with cancer.

  • bacteria – are associated with bladder infections, which can cause signs of weakness in older animals.


(d) Serum thyroxine (total T4) is a hormone produced by the thyroid gland. The level of T4 in the blood should be evaluated in both cats and dogs that are showing signs of weakness.

In cats, weakness may develop due to a condition called hyperthyroidism in which the thyroid gland makes too much thyroid hormone. This leads to breakdown of skeletal muscle (seen as weight loss), as well as a drop in blood potassium concentration, causing weakness. A diagnosis of hyperthyroidism can often be made with a single blood sample.

In dogs, weakness may develop due to a condition called hypothyroidism, in which the thyroid gland does not produce enough thyroid hormone. Weakness develops due to anemia, low metabolic rate, and poor muscle tone. If the Total T4 measurement is low, then hypothyroidism may be present and additional testing is recommended to confirm the diagnosis.

What additional tests might be recommended for my pet showing weakness?

There are many additional tests that might be recommended depending on the results of your pet’s history, physical examination, and screening tests. Some examples of further testing would include:

  • additional testing to assess kidney or liver function: if liver or kidney values are abnormal

  • imaging studies: X-rays and ultrasound to diagnose heart and lung disease, spinal cord disease, tumors in the chest or abdomen

  • tests for heart disease: electrocardiogram, echocardiogram, and blood tests to measure heart muscle damage

  • tests for anemia: Coomb’s test, serum iron, fecal occult blood, parasite tests, bone marrow aspiration

  • parathyroid hormone (PTH) testing: if your pet has low calcium levels in the blood. Parathyroid hormone controls the level of calcium in the blood

  • examination of joint fluid: in pets where joint disease may be contributing to your pet’s weakness

  • ACTH stimulation test: if Addison’s disease (adrenal gland failure) is suspected

  • specialized testing: for rare disorders such as myasthenia gravis (immune-mediated disorder of the nerves) and pheochromcytoma (adrenal cancer).

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