The clinical signs of hyperthyroidism in cats can mimic those of kidney disease, liver disease, diabetes and cancer. Diagnosis is based upon a thorough physical examination and history, the cat’s presenting signs, a complete blood panel and blood tests to detect the levels of circulating thyroid hormones. In some cases, imaging techniques are helpful as well.
Diagnosing Hyperthyroidism in Cats
Physical Examination and History
Hyperthyroidism causes a classic series of signs in cats. Most veterinarians begin their diagnostic journey by getting a complete description from the owner of what unusual clinical signs they have noticed in their cat, followed by a thorough physical examination. Typically, the doctor will be able to feel (palpate) enlarged lobes of the thyroid gland which are located on each side of the cat’s trachea.
A complete blood work-up, consisting of a complete blood count and serum biochemical profile, together with a urinalysis, are usually a standard part of the diagnostic process for cats that seem to have some systemic disease. Additional blood tests are available to rule in or rule out hyperthyroidism. The advanced test that normally is done first in cats suspected of having hyperthyroidism is called a serum total T4 concentration test. This measures the total amount of circulating thyroxine hormone in the blood and often is diagnostic of hyperthyroidism if it is consistently elevated. Other blood tests are available to measure the amount of other thyroid hormones in circulation. Taken together, the results of these tests will help the veterinarian make a definitive diagnosis. Unfortunately, some cats with mild hyperthyroidism have normal blood test results, and in these cases additional testing may be necessary.
Thoracic radiographs, and possibly an electrocardiogram and/or echocardiogram, are frequently part of the diagnostic work-up in cats suspected of having hyperthyroidism – mainly to assess heart size, the presence and cause of cardiac arrhythmias and generally to assess the severity of heart disease. In some instances, thyroid imaging is used in the diagnosis of hyperthyroid cats. This technique, called thyroid gland scintigraphy, is only available at certain specialized veterinary facilities and usually is only performed in cases where the hyperthyroidism is thought to be caused by cancer, or in cases where the attending veterinarian deems it valuable to visualize how much of thyroid gland has been affected by the disease.