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Malignant Thyroid Tumors in Dogs and Cats

Date Published: 10/28/2007
Date Reviewed/Revised: 03/17/2021
Original illustration by MarVistaVet

Meet the Thyroid Glands

The two thyroid glands are located in the throat, nestled just behind the larynx on either side of the windpipe. These glands are responsible for making and storing thyroid hormones, in particular thyroxine which is also called T4. This hormone is released and then activated by the body’s tissues into triiodothyronine, or T3. Triiodothyronine (T3) acts as a sort of a volume dial for metabolism, controlling how fast our cells burn fuel, while thyroxine (T4) is an inactive form. There is actually quite a long story involving how these hormones work, how an assortment of inactive forms are made and shifted back to activity again, but that story is for another time. Today’s story is not about the healthy thyroids and their activity but about thyroid malignancy. The situation is a little different between dogs and cats so we’ll discuss them separately.


The thyroid tumor in dogs generally does not affect production of thyroid hormones. This means there is nothing to detect on a screening lab report that would indicate a tumor; a dog with a thyroid tumor is usually neither hypothyroid nor hyperthyroid. The tumor is usually detected when someone, usually either the owner at home or the veterinarian during examination, notices a lump around the dog’s throat. The lump may be deeply adhered, potentially altering the dog’s voice or ability to swallow, or the lump may be fairly moveable. The average age at tumor detection is 9 years and there is no gender predilection. Some studies have suggested that the following breeds are predisposed to developing thyroid carcinoma: Boxer, Beagle, and Golden Retriever.

In canine thyroid carcinomas, approximately one dog in five will be hyperthyroid.

So what happens after a lump is detected?

Canine thyroid carcinoma imaged by scintigraphy. The dog's nose is pointing upward. The two red spots are the salivary glands (which take up i-131) and the large red spot is the tumor. The image shows the dog's head and neck. Image provided by Michael R. Broome, DVM, MS, Dipl. ABVP and AVMI

The goal at this point is to assess the dog for general health and for potential tumor spread as well as to identify the lump. The initial assessment of a dog with a throat lump should include a physical examination, basic database of blood and urine testing including a T4 level, and a set of chest radiographs (x-rays) to check for obvious tumor spread. Testing will be needed beyond this to identify the lump/growth.

There are several procedures that could be performed to identify the growth. If the growth is loose and fairly moveable in an accessible area, the easiest procedure may be to simply remove and biopsy it. This provides the most accurate means of identifying the growth and may even prove curative. The problem is that often the thyroid carcinoma is extremely vascular and deeply adhered to the deeper structures of the throat, which means that surgery may not be so simple.

A less invasive option is a fine needle aspirate. Here, a needle is inserted in the growth and cells are withdrawn with a syringe. A sample is submitted to the lab for analysis. This is not nearly as accurate as a biopsy when it comes to distinguishing benign from malignant thyroid disease but greater than 90% of the time this procedure will at least identify the growth as thyroid in origin. Since statistics indicate a thyroid growth is malignant 87% of the time, this goes a long way to identifying it as a thyroid carcinoma. Cytology only detects malignancy directly in about 50 percent of the samples where the tumor later proved to be malignant; in most cases, we can infer malignancy simply by knowing that the cytology confirmed thyroid cells.

In dogs, there is an 87% chance that a thyroid growth is malignant.

How do we treat a thyroid carcinoma after we are pretty sure that’s what it is?

Treatment depends on several factors: how deeply invasive the tumor is and whether there is already measurable tumor spread. There are three imaging techniques that can be used, and which are sometimes combined, to answer these questions: ultrasound, scintigraphy (nuclear medicine), and MRI.

Ultrasound uses sound waves to image the deeper structures of the throat and determine the feasibility of removing the primary tumor. This method is non-invasive and many hospitals have the necessary equipment available. If surgery is being considered, ultrasound might be an especially helpful choice as it will indicate whether surgical resection is even possible.

Scintigraphy is especially helpful at detecting tumor spread. Here, a radioactive material called pertechnetate is given intravenously. It follows a similar course as iodine would, iodine being an important component of thyroid hormone. This means that the nuclear medicine image can reveal areas of thyroid tissue in all sorts of inappropriate areas indicated the extent of tumor spread. Scans are reasonably priced but not necessarily available in all areas; one has to be lucky enough to live near nuclear medicine facility.

MRI (magnetic resonance imaging) is another way of detecting the depth of a tumor and possibly tumor spread. A highly detailed image is produced but imagining requires general anesthesia and is expensive. Again, not every area has a facility that can perform this kind of imaging, but potentially one could be in an area with MRI access but without scintigraphy access, so an MRI might come to be a helpful tool in tumor assessment.

Studies show that 16% to 38% of dogs with thyroid carcinoma have evidence of tumor spread at the time of diagnosis.

Larger tumors are more likely to spread faster than smaller tumors.

When the thyroid carcinoma spreads, it usually goes to the lymph nodes of the throat and neck and to the lungs.


After the tumor has been assessed for spread and for depth, an appropriate therapy can be selected.

If the tumor is freely moveable and no tumor spread can be detected, surgery is probably the best choice. A median survival time of 20.5 months was reported in one retrospective study of such cases. If, however, the tumor is invasive and not mobile, excision will probably not be complete and will likely have complications such as nerve damage or excessive bleeding. It is best in this situation to pursue a different therapy.

External Beam Radiation Therapy
This form of therapy requires referral to a special radiation oncologist and such facilities may not be readily available. In this therapy, the primary tumor in the throat is irradiated, typically three times weekly. The tumor responds slowly (peak effect is usually seen 8 to 22 months after the treatment course has been completed). In one study of eight dogs, a median survival time of 24.5 months was found. Side effects include hoarse voice, cough, and difficulty swallowing (usually complications were self limiting and resolved in 2-3 weeks). Later complications include baldness at the radiation site and dry cough in fewer than 20% of patients.

Radioiodine Therapy
As in hyperthyroid cats with benign thyroid nodules, iodine 131 can be used to treat thyroid carcinoma in dogs. The advantage of this therapy is that the iodine can travel to areas of tumor spread and be sequestered there. Iodine 131 emits beta particles (high speed electrons) penetrating and destroying tissue for only a fraction of an inch. This means the normal tissue is spared around the tumor and only the tumor is treated. In one study, this therapy yielded a median survival time of 30 months. Much higher doses of iodine 131 are used in this situation than in the feline treatment of hyperthyroidism. This translates to most dogs requiring thyroid supplementation at the end of therapy as there will probably not be enough normal thyroid tissue left. There is a less than 10% incidence of fatal bone marrow suppression associated with this therapy. Dogs must stay at the radiofacility during their course of therapy and afterwards until their radiation emissions drop to a safe level. A special diet is sometimes recommended prior to therapy to maximize the uptake of iodine 131 so as to enhance the tumor’s response.

Chemotherapy is still under investigation as a treatment for thyroid carcinoma but protocols are still being worked out. There is some evidence that treatment with non-steroidal anti-inflammatory drugs (NSAIDs) suppress progression of many types of carcinomas. Use of these medications may be a helpful adjunctive treatment.


Bilateral asymmetric adenomas in a cat. We are looking down on the cat's front quarters as an aerial view. The cat's nose faces the top of the frame. The salivary glands appear as symmetrical red spots. The tumor is the large white spot in the area of the neck. The front legs can be seen alongside the neck. Image provided by Michael R. Broome, DVM, MS, Dipl. ABVP and AVMI

While canine thyroid carcinomas do not usually produce excess hormone, the opposite is true in cats; cats with thyroid carcinomas are usually hyperthyroid. Hyperthyroidism is an extremely common situation in older cats and the thyroid growth involved is benign in over 95% of cases. This makes finding the less than 5 percent of cats with malignancy all the more difficult to distinguish.

A pertechnetate scan is needed to make the distinction. If the owner is considering radiotherapy for treatment of hyperthyroidism, the malignancy should be identified as the scan normally precedes radiotherapy. If the owner is opting for another treatment (surgery, or oral medication), the scan may not happen and the diagnosis will be missed.

In most cases when cats receive radiotherapy for their hyperthyroidism, a pertechnetate nuclear medicine scan (scintigraphy) is performed to confirm the diagnosis and to determine the dose of iodine 131 needed to control the thyroid disease. If the thyroid growth is malignant, there are generally clues on the scan suggesting that the tumor mass is more extensive than the more routine benign thyroid growths. Assorted image criteria are used to make this determination but, of course, there is no substitute for harvesting an actual tissue sample.

So when would you consider such a biopsy given that over 95% of thyroid growths are benign? If the standard hyperthyroidism therapies (medication or radiotherapy) fail to control thyroid levels, then there is reason to pursue malignancy as the explanation. Obviously, if the hyperthyroid cat is treated with surgical removal of the thyroid glands (rarely done nowadays given the availability of radiotherapy and medication), then the tissue would be biopsied to definitively settle the benign/malignant question. Right now, the treatment of hyperthyroidism generally consists of a choice between radiotherapy, medication, and possibly dietary therapy. The scan done in conjunction with radiotherapy is probably the best way to identify the small group of cats that have malignant disease.

Since cats with benign or malignant thyroid tumors are generally hyperthyroid, it should not be surprising that radiotherapy is probably the best treatment in either situation. Thyroid tumors readily take up radioiodine regardless of whether they are benign or malignant. In one study, cats with malignant thyroid tumors who received radioiodine therapy not only were cured of their hyperthyroidism but also the median survival from their cancer was approximately three years. In cats with thyroid tumors who were not also hyperthyroid, treatment is a little trickier. The pertechnetate scan is helpful in determining the tumor's ability to absorb radioiodine. If the tumor is not going to take up radioiodine, then some other therapy (surgery or external beam radiation) is a better choice.