Hyperthyroidism in Cats

The thyroid gland is located in the neck and secretes hormones involved in regulatingHyperthyroidism in Cats metabolism. Hyperthyroidism in cats is caused by excess secretion of these hormones, causing an increase in overall  metabolism. About 95% of cats with hyperthyroidism will have a benign tumor, and the remainder of cats with hyperthyroidism with have a cancerous tumor. Hyperthyroidism typically affects geriatric cats, usually cats over the age of 8 years.

The exact cause of hyperthyroidism in cats is unknown. Males and females are equally affected and there has been no breed predisposition found. The incidence of hyperthyroidism in cats has increased dramatically over the past few decades. Hyperthyroidism is the most common endocrine disorder in cats. It can be seen in 1 in 50 cats over 10 year of age in some estimates. Hyperthyroidism in cats is more prevalent in the United States than many other countries. Theories behind this difference include increased use of flame retardant and other toxins or increased use of canned foods within the U.S.

Clinical signs of hyperthyroidism in cats

The main clinical signs of hyperthyroidism in cats are an increased appetite and weight loss.  Other clinical signs of hyperthyroidism that can occur are an increased thirst and urination, vomiting, diarrhea, and alterations in behavior (often an increase in vocalization or hyperactivity). A very small percentage of cats with hyperthyroidism will actually develop a decreased appetite, known as apathetic hyperthyroidism. Hyperthyroidism can cause high blood pressure and heart disease, hypertrophic cardiomyopathy, that can lead to congestive heart failure. Signs of heart failure are increased respiratory rate and effort, and eventually respiratory distress.  High blood pressure can cause seizures and blindness due to retinal detachment.

Diagnosis and treatment of hyperthyroidism in cats

Diagnosis of hyperthyroidism in cats can be made by physical examination and bloodwork. On physical examination a cat with hyperthyroidism will often have noticeable weight loss and an enlarged thyroid nodule that can be palpated by a veterinarian.  A blood sample can be submitted to check the thyroid hormone level (T4) and it will be elevated in a cat with hyperthyroidism.   In most cases of hyperthyroidism in cats the diagnosis can be made by evaluating the total T4 level, but if this test does not confirm hyperthyroidism, then a free T4 level can be evaluated.  The free T4 level is the active portion of T4 and is more accurate in assessing the thyroid function.

Once the diagnosis has been made, treatment for hyperthyroidism can be initiated. Methimazole is a medication that cats can take orally or transdermally (through the skin) that reduces the secretion of thyroid hormones from the tumor. This medication does not affect the thyroid tumor, but it can normalize the thyroid hormone level.  Cats often tolerate this medication, but potential side effects of methimazole include bone marrow suppression, liver disease, facial excoriations, gastrointestinal upset, or unmasking kidney disease.  If any of these side effects occur the methimazole can be discontinued and the side effects are often reversible.  Hyperthyroid cats on methimazole should be evaluated by their veterinarian and have bloodwork checked to monitor kidney and liver function, and a blood count.  Another treatment option for a cat with hyperthyroidism is surgical removal of the thyroid mass.  The surgery is often not difficult but there is a chance for recurrence.  The last treatment option, which is often curative, is administration of radioactive iodine, called I-131. The radioactive iodine attacks only the hyperactive thyroid tissue and does not cause side effects. Cats receiving radioactive iodine must be kept in a special facility during the treatment for a few days to over a week while the radioactive iodine level in their body decreases to acceptable levels.

Click here to learn about thyroid disease in dogs.

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Dr Peter Nurre

Peter Nurre, DVM, Diplomate ACVIM (internal medicine) is a board- certified veterinary internal medicine specialist. His professional interests include internal medicine and cardiology.

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