Hyperthyroidism Is Common in Senior Cats
Hyperthyroidism (also called thyrotoxicosis) is one of the most common diseases of the middle-aged and older cat. It is a multi-system disorder caused by an increase in the amount of thyroid hormones (called T3 and T4) produced by an enlarged thyroid gland. It was first documented in cats almost 30 years ago but the cause of the disease has been elusive. Although the enlargement in the thyroid gland is caused by a tumor, called an adenoma, it is non-cancerous.
The most common clinical signs of hyperthyroidism in cats include weight loss, increased appetite (although some patients have decreased appetite), vomiting, increased thirst and urination, hyperactivity, and diarrhea. The high levels of thyroid hormones can cause the development of heart disease, and these patients may have a heart murmur, difficulty breathing, high heart rate and arrhythmias.
Veterinarians will order a blood chemistry panel as well as a thyroid hormone (T4) level in cats suspected of being affected by this disease. It is important to evaluate the health of the other major organs, including the kidneys and heart in these patients. Typically, hyperthyroid cats may have elevations in their liver enzymes. Chest x-rays and cardiac ultrasound may reveal secondary hypertrophic cardiomyopathy. Generally, the cardiac changes will reverse when the hyperthyroidism is treated. In some cases, specific heart medication may be needed to stabilize cardiovascular health. Some cats with hyperthyroidism also have systemic hypertension (high blood pressure). This can be readily diagnosed and treated. In recent years, it has been recognized that many hyperthyroid cats have concurrent chronic kidney insufficiency that is being masked by the effects of hyperthyroidism. Treatments directed at curing hyperthyroidism in these patients could lead to a worsening of their kidney function.
Most hyperthyroid cats will have elevated levels of the thyroid hormone T4 in their bloodstream on a routine screening test. However, a small percentage of hyperthyroid cats will have normal T4 levels. If hyperthyroidism is still strongly suspected in these patients, other tests such the T3 suppression test or the free T4 test can be performed to confirm the diagnosis. If available, a type of imaging of the thyroid gland using radioisotopes called thyroid scintigraphy is also helpful with diagnosis.
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