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Thyroid Cancer in Dogs and Cats

What is thyroid cancer (adenocarcinoma)?

 The thyroid glands are paired structures located along the windpipe (trachea), about halfway down the neck of dogs and cats.

The thyroid glands produce hormones that are vital for normal body function. Thyroid growths can be benign (adenoma) or malignant (carcinoma). Benign growths tend to get larger and may produce excess hormones; malignant growths can also spread to other parts of the body. While benign tumors of the thyroid gland are common in cats, the majority of dogs that have tumors have malignant tumors. Thyroid tumors are commonly seen in middle-aged to older large-breed dogs such as boxers, beagles, golden retrievers and Siberian huskies. They account for only 1.2–3.8% of all canine tumors.

Signs and Symptoms

Here are the symptoms commonly related to adenocarcinoma of the thyroid:

  • A large fixed or movable mass in the neck
  • Difficulty breathing
  • Difficulty swallowing
  • Weight loss
  • Hoarseness/change in bark
  • Increased thirst
  • Increased amount and/or frequency of urination.

It is rare for a tumor to affect the hormonal function of the thyroid. However, it can happen, and in those cases you will see additional symptoms. With hypothyroidism, your pet could be lethargic, have some hair loss, and suffer from exercise intolerance. In the case of hyperthyroidism, dogs can exhibit signs including heart trouble (rapid heart rate and abnormal rhythm), increased hunger and thirst, and muscle tremors.

Diagnosis and Staging

The common diagnostic techniques used to detect thyroid cancer include thorough physical examination, complete blood count, serum chemistry panel, urinalysis, (three-view) thoracic radiographs, and cytologic or histologic evaluation of the regional lymph nodes. Advanced imaging (ultrasound or CT or MRI) is sometimes advised, to determine the extent of the disease. Ultimately, a tissue biopsy may be necessary in order to make a definitive diagnosis. Undergoing these tests will help to evaluate the stage that the disease has reached.


The course of treatment will depends upon the size of the tumor, the extent of pervasiveness, the presence or absence of metastasis, and whether there are signs of thyrotoxicosis (a condition in which the thyroid gland produces excessive hormones).

Surgical removal of cancerous thyroid tumors is often difficult because they can invade local blood vessels or other tissues. Therefore, radiation or chemotherapy is most often recommended for those masses that are incompletely removed or are too large for successful surgery.

Stereotactic radiosurgery (SRS) is an advanced form of radiation therapy that has been proven effective in treating humans with cancer, including thyroid cancer. SRS is designed to cure cancer, not just ease the symptoms. What sets SRS apart is its unprecedented precision, which enables destruction of the tumor with minimal damage to the healthy cells around it. SRS is a noninvasive therapy that puts a premium on your pet’s quality of life. There are far fewer treatment sessions, anesthetic events and side effects, compared to conventional radiation therapy.

Radioactive iodine (I-131) treatment has often been shown to be an effective adjunct to the treatment on thyroid tumors. I-131 can be utilized in patients that are poor surgical candidates or where vascular invasion has been identified in spite of surgical removal.

A comprehensive examination of the removed tumor is important, to determine if any additional treatments (chemotherapy, radiation therapy, I-131 treatment, etc.) would be beneficial.


Surgical removal of thyroid tumors has the best outcome if the mass is freely movable, less than 4cm in size, has not spread and can therefore be completely removed. Long-term survival (1 to 3 years) may be achieved in both dogs and cats. It is common for medications to be needed after surgery. If both thyroid glands are removed, your veterinarian may also need to check your pet’s calcium levels several times during recovery. This is because some parathyroid tissue is removed with the thyroid glands, and parathyroid glands play a role in calcium regulation.

If surgery is not possible, then SRS may be a viable and successful option.

Meet our Pet Hero – Thyroid Cancer Survivor Liza!

We’d like you to meet our Pet Hero Liza, a 10-year-old Collie who was successfully treated for thyroid cancer at PetCure Oncology in Cincinnati. Early in 2015, Liza began having health issues that prompted a trip to see the family veterinarian. Liza’s parents explained that she was coughing frequently and seemed to be trying to clear her throat. Her bark had also changed, becoming hoarse and dry. Over time, Liza’s breathing became strenuous and the family felt a lump on her neck….read more

Questions? We are here to help.

No one knows your pet better than you do. If you see any possible signs of thyroid cancer, do not hesitate to contact your primary care veterinarian for a comprehensive evaluation. As with any illness, early awareness and detection is the key to successful treatment.

If you would like additional information on cancer care or SRS, please contact PetCure Oncology at 773.850.3400.


Published September 19, 2017 | By PetCure Oncology | Tagged , , , , , | Return to Blog.