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Testing for Weakness
How can I tell if my pet is feeling weak?
Pets that feel weak often have difficulty getting on 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 intermittent. 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. Some common causes of weakness include:
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 should report any changes you have noticed in your pet’s activity, attitude, appetite, water intake, bowel habits, and urination. It is also important to tell your veterinarian if your pet is taking medications and if you have seen your pet eat anything unusual.
A 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). 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 severe organ failure; an abdominal mass could signal a tumor; hair loss could be a sign of hypothyroidism in a dog.
History and the 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 your pet’s overall health 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). Additional tests may be recommended based on the results of these screening tests.
Each screening test provides additional information about your pet.
This simple blood test 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 clot. The CBC provides details about the number, size, and shape of the various cell types and identifies abnormal cells in circulation.
In a pet with signs of weakness, changes seen on a CBC could include:
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 (salts and minerals in the blood), 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:
Urinalysis is a routine test that analyzes 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 weak pet, the urinalysis might show changes such as:
Serum thyroxine (total T4) is a hormone produced by the thyroid gland. The level of T4 in the blood should be evaluated in cats and dogs showing signs of weakness.
In cats, weakness may develop due to hyperthyroidism, in which the thyroid gland makes too much thyroid hormone. This leads to the breakdown of skeletal muscle (seen as weight loss) and 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 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 is low, hypothyroidism may be present and additional testing is recommended to confirm the diagnosis.
Depending on the results of your pet’s history, physical examination, and screening tests, many additional tests might be recommended, including:
© Copyright 2025 LifeLearn Inc. Used and/or modified with permission under license. This content written by LifeLearn Animal Health (LifeLearn Inc.) is licensed to this practice for the personal use of our clients. Any copying, printing or further distribution is prohibited without the express written consent of LifeLearn. This content does not contain all available information for any referenced medications and has not been reviewed by the FDA Center for Veterinary Medicine, or Health Canada Veterinary Drugs Directorate. This content may help answer commonly asked questions, but is not a substitute for medical advice, or a proper consultation and/or clinical examination of your pet by a veterinarian. Please contact your veterinarian if you have any questions or concerns about your pet’s health. Last updated on Oct 3, 2022.
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