Thyroid tumors are fairly rare in dogs but relatively common in middle-aged to older cats.  In dogs, thyroid tumors are usually malignant; in cats, they are usually benign.  These tumors can often be felt as small lumps in the center of the throat area, although a diseased gland can sometimes creep down the neck so that the lumps are lower or pushed to the side.

Affected cats usually develop hyperthyroidism, because the tumorous glands produce too much thyroid hormone.  Cats with this condition have an increased appetite, drink more water (and urinate more), lose weight, and often seem hyperactive or even agitated.  As their metabolism speeds up, they also tend to breathe more rapidly and to have a very high heart rate, even at rest.

To diagnose a thyroid tumor, your veterinarian will want to take a blood sample to check the levels of thyroid hormone.  Most affected cats have high levels, but a small percentage will have normal thyroid levels even in the presence of a tumor.  Your vet may also want to take x-rays or run other blood tests (e.g., a complete blood count or a feline leukemia test) to rule out infection or other causes.

Thyroid tumors can be treated by surgically removing the thyroid gland or via injection of radioactive iodine.  Surgery must be done carefully to avoid possible damage to nearby structures, such as the parathyroid glands, which have an important role in calcium metabolism.  Radioactive iodine administered intravenously concentrates within the thyroid gland, killing the tumor cells, while sparing the rest of the body the effects of radiation.  Removal of both lobes of the thyroid gland (either via surgery or radioactive iodine) means that your cat will need to take daily thyroid supplements to replace the hormone that is no longer being produced. Both these procedures involve specialized expertise, so your vet may refer you to a specialty practice or veterinary teaching hospital.

Another option is to leave a benign thyroid tumor in place, but control symptoms by decreasing the secretion of thyroid hormone.  This involves giving an antithyroid medication called methimazole, which blocks production of thyroid hormone within the diseased gland.  Methimazole must be given daily for the rest of the cat's life, with periodic monitoring to measure thyroid hormone levels in the blood.

If your cat is diagnosed with a thyroid tumor, you should discuss the various treatment options available to make the best choice for your pet.  Most cats with thyroid tumors can be treated effectively, and they generally do quite well.

Q&A

How can I tell if my cat has a thyroid tumor?

Affected cats usually exhibit signs of hyperthyroidism, which include increased appetite, weight loss, and hyperactivity.  These tumors can often also be felt in the middle of the throat.

Can dogs develop these tumors?

Yes, but this is rare.  Thyroid tumors in dogs are usually malignant.

How are thyroid tumors treated?

Feline thyroid tumors can be treated by surgically removing the thyroid glands or via injection of radioactive iodine.  Another option is to control symptoms by administering an antithyroid medication called methimazole, which inhibits secretion of thyroid hormone.

What is the prognosis for cats with thyroid tumors?

Most do well after the thyroid is removed, but lifelong daily treatment with thyroid supplements is required.