Thyroid Tumors

By Christopher Pinard, DVM

Tumors, Pet Services

What is the thyroid?

The thyroid is a two-lobed gland that lies along the trachea (windpipe) in the neck region. It produces a specialized chemical called thyroid hormone. Thyroid hormone is responsible for many functions in the body, including how the body uses energy (the metabolic rate) and how the body responds to other hormones.

What is a thyroid tumor?

A thyroid tumor develops as a result of abnormal replication or growth of the cells that make up the thyroid gland; specifically, the glandular tissues. Benign (non-cancerous) thyroid tumors are referred to as adenomas, while malignant (cancerous) thyroid tumors are referred to as carcinomas and adenocarcinomas. In dogs, most thyroid tumors (about 90%) are malignant but nonfunctional, meaning they do not cause excessive thyroid hormone production.

"A thyroid tumor develops as a result of abnormal replication or growth of the cells that make up the thyroid gland; specifically, the glandular tissues."

In contrast, in cats, even though most tumors are benign, they are functional, meaning they produce and cause an excess of circulating thyroid hormone, a condition called hyperthyroidism. Only about 1-3% of cats have malignant tumors, and these, too, cause hyperthyroidism.

It is quite common for dogs and cats to have thyroid tissue elsewhere in the body (called ectopic thyroid tissue). Ectopic thyroid tissue may be under the tongue, further down the neck, or all the way down to the base (or top) of the heart. Ectopic thyroid tissue can also develop tumors, so thyroid tumors can develop in a range of locations in the body.

What causes this type of cancer?

The reason why a particular pet may develop this, or any tumor or cancer, is not always straightforward. Very few tumors and cancers have a single known cause. Most seem to be caused by a complex mix of risk factors. With thyroid tumors, although there are no known dietary or environmental causes, there appears to be a genetic component, as certain breeds are more commonly affected, including Golden Retrievers, Beagles, Boxer Dogs, and Siberian Huskies. No breed predisposition has been identified in cats.

What are the signs of a thyroid tumor?

Dogs with thyroid tumors may have no signs whatsoever, or only a mass on the underside of the neck. However, if the mass compresses the windpipe (trachea), it may cause coughing or difficulties breathing (rapid breathing or shortness of breath), or if it presses on the esophagus, it may cause gagging or problems with swallowing. Some dogs may have facial swelling, a change in their bark, loss of appetite, or weight loss.

"Dogs with thyroid tumors may have no signs whatsoever, or only a mass on the underside of the neck."

In dogs, it is uncommon for a tumor to affect the hormonal function of the thyroid, but it is possible, and in those cases, other symptoms may be seen. With functional tumors and hyperthyroidism, your dog could develop heart problems (rapid heart rate and abnormal rhythm), and have increased hunger and thirst, and muscle tremors. If the cancerous tissue destroys too much of the normal tissue, the opposite of hyperthyroidism, hypothyroidism (low thyroid hormone) can result, and your dog could become lethargic, tire easily, and have hair loss.

Cats with thyroid tumors may also have a palpable mass on the underside of the neck, but most of the signs are related to the overproduction of thyroid hormone. Hyperthyroidism can cause a wide range of symptoms, including weight loss, increased appetite, increased activity and/or vocalization, increased heart rate, a heart murmur, vomiting, diarrhea, increased water consumption, and increased urination. It can also cause changes in behavior. Cats may groom themselves less than usual (the coat becomes unkempt, matted, or greasy) and may become restless, cranky, or even aggressive. Some cats will show all of these signs, but many will show just one or two.

"Cats with thyroid tumors may also have a palpable mass on the underside of the neck, but most of the signs are related to the overproduction of thyroid hormone."

Approximately 10% of hyperthyroid cats have an unusual form of the disease, called apathetic hyperthyroidism. In these cats, the typical signs of hyperactivity and increased appetite are replaced by depression and inappetence. Because these cats do not eat well, they also lose weight.

How is this type of tumor diagnosed?

This type of tumor is diagnosed starting with a thorough physical examination and bloodwork. Your veterinarian may find a palpable thyroid mass as well as changes in your pet suggestive of thyroid hormone excess (e.g., an increased heart rate) or deficiency (e.g., hair loss). Bloodwork will include an analysis of the level of thyroid hormone, T4.

In most cases, the finding of an increased T4 in cats is sufficient to establish a diagnosis. At times, however, other procedures, such as diagnostic imaging, may be recommended, including thyroid scintigraphy (which is especially useful to detect ectopic thyroid tissue), ultrasound, CT (computed tomography) or MRI (magnetic resonance imaging).

"In most cases, the finding of an increased T4 in cats is sufficient to establish a diagnosis."

With dogs, a tissue biopsy is usually necessary to make a definitive diagnosis. Since most tumors in dogs are malignant and spread locally or to other areas of the body (metastasize), advanced imaging (ultrasound or CT or MRI) is commonly required to determine the extent of the disease. Staging is the process of determining the extent to which a cancer has grown and spread. Your veterinarian may recommend staging, which, in addition to advanced diagnostic imaging, may include full bloodwork, urinalysis, and chest X-rays. Given that these tumors are malignant, the local lymph nodes may also be sampled to determine if the tumor has spread. About 40-50% of dogs have evidence of spread to other body tissues at the time of diagnosis.

How do thyroid tumors typically progress?

Left untreated, cats with hyperthyroidism will grow very sick over the years. Excess thyroid hormone affects virtually every organ in the body. As metabolism increases, and the heart works harder, cats can develop heart disease, high blood pressure, and eventual heart failure; kidney disease; chronic wasting of the body; and blindness. Left untreated, hyperthyroidism can lead to death.

As most thyroid tumors in dogs are malignant, left untreated, they will continue to grow locally, infiltrating nearby tissues. Approximately 65% to 90% of dogs will eventually develop metastasis to a wide variety of body tissues.

What are the treatments for this type of tumor?

A variety of treatment options are available for dogs with thyroid tumors. The choice of treatment will depend upon the size of the tumor, the extent to which it has invaded nearby tissues, the presence or absence of metastasis, and the treatment options available. Surgery, chemotherapy, radiation, and the use of radioactive iodine therapy, alone or in combination, may be indicated depending on the individual.

"A variety of treatment options are available for dogs with thyroid tumors."

Surgery (called a thyroidectomy) is recommended for dogs with movable (loosely attached) thyroid tumours and tumours with limited invasion of the surrounding tissues. For tumors that are too large for surgery, deeply invasive or firmly attached, or not able to be completely removed, radiation or chemotherapy is recommended.

Another treatment option is the use of radioactive iodine (I-131). This is similar to the treatment used in people in which a radioactive tag is placed on iodine, and once taken up by the thyroid gland (that processes iodine) slowly destroys the cancerous thyroid tissue. I-131 can be used in dogs that are poor surgical candidates or when metastasis has been found after surgery. The disadvantages of I-131 therapy are the need for very high doses (compared to cats) and an extended hospital stay.

There are four treatment options for cats with thyroid tumors including medication, radioactive iodine therapy (I-131), surgery, and dietary therapy. When available, radioactive iodine therapy is the treatment of choice, as it is simple, effective, and safe. Using a daily medication (e.g., methimazole) is also safe and effective. Thyroidectomy is reserved for cats when medication is not possible, not effective, or has unacceptable side effects, and when radioactive iodine therapy is not available. A newer treatment is the dietary restriction of iodine using a prescription diet.

"There are four treatment options for cats with thyroid tumors including medication, radioactive iodine therapy (I-131), surgery, and dietary therapy."

These treatment options have different pros and cons and should be discussed with your veterinarian. Not all treatment options are suitable for all patients. It is important to note that surgery may be an option for some ectopic tumors in dogs and cats, especially with sublingual (below the tongue) tumors. For other ectopic tumors, such as those in the chest cavity, where surgical removal is difficult, methimazole or radioactive iodine treatment may be used.

Is there anything else I should know?

With appropriate treatment, the prognosis for dogs with thyroid tumors is good to excellent. Even dogs that are not surgical candidates can enjoy long survival times when treated appropriately. The prognosis for cats is also good with appropriate therapy. Generally, the earlier the diagnosis and treatment, the better the outcomes.

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