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Understanding Thyroid Cancer in Dogs: Signs, Diagnosis, and Treatment Options

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Thyroid cancer is a condition that affects the thyroid gland in dogs. The thyroid gland, located in the neck, plays a crucial role in regulating metabolism through the production of thyroid hormones. Thyroid tumors can be either benign (non-cancerous) or malignant (cancerous), with the majority of thyroid tumors in dogs being carcinomas. In this article, we will explore the signs, diagnosis, and treatment options for thyroid cancer in dogs.

Thyroid Cancer in Dogs

Signs of Thyroid Cancer in Dogs

The most common sign of thyroid cancer in dogs is the presence of a lump or mass in the neck area. Other signs may include voice changes, difficulty swallowing, or rapid, harsh breathing. Hyperthyroidism, characterized by excessive production of thyroid hormones, can lead to weight loss and coat changes. On the other hand, hypothyroidism, which is the underproduction of thyroid hormones, can cause lethargy, weight gain, and coat changes.

Diagnosing Thyroid Cancer

To diagnose thyroid cancer in dogs, a veterinarian may perform a fine needle aspirate or a biopsy to examine the tumor cells. However, due to the extensive blood supply of thyroid tumors, these procedures may carry an increased risk of hemorrhage. Imaging techniques such as ultrasound, CT scans, or MRI scans can help evaluate the invasiveness of the tumor and determine the potential for surgical removal. Bloodwork and lung imaging may also be recommended to check for metastasis (spread of the tumor).

Treatment Options for Thyroid Cancer

The treatment options for thyroid cancer in dogs depend on various factors, including the size of the tumor, its invasiveness, the presence of metastasis, and the signs of thyrotoxicosis. Here are the main treatment options available:


Surgical removal of the tumor is recommended for dogs with mobile thyroid tumors and those with limited invasion. However, if the tumor is deeply invasive or fixed to surrounding tissues, complete removal may not be possible, and surgery is not recommended.

Radiation Therapy

Radiation therapy is often recommended for dogs with fixed or invasive thyroid tumors or those with tumors that couldn’t be completely removed surgically. Stereotactic radiosurgery (SRS), an advanced form of radiation therapy, has shown effectiveness in treating thyroid cancer in humans and can be considered for dogs as well. SRS offers precise tumor destruction while minimizing damage to healthy cells, resulting in fewer side effects and a better quality of life for the pet.

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Radioactive Iodine Therapy

Radioactive iodine therapy, primarily used in cats, may also be considered for dogs with thyroid tumors. However, this treatment option is limited by the availability of facilities and limited data on its effectiveness in dogs.


Chemotherapy may be recommended for dogs with larger, bilateral, or invasive thyroid tumors due to the high risk of metastasis. Response rates of 30-50% have been observed with chemotherapy, and there are various approaches and options that can be discussed with an oncologist.

Prognosis and Survival Rates

The prognosis for dogs with thyroid cancer is generally good when appropriate therapy is administered. Untreated dogs with malignant thyroid tumors have an average survival time of only three months. However, dogs with small, unilateral thyroid tumors that undergo surgery can have median survival times of over three years. Dogs with fixed thyroid tumors have a 25% survival rate at one year post-surgery, while those treated with radiation therapy have reported median survival times of more than two years. Even dogs with evidence of lung metastasis at the time of diagnosis have achieved long survival times.

Radiation Therapy for Thyroid Tumors

Definitive radiation therapy for thyroid tumors typically involves daily treatments over a period of three to four weeks, excluding weekends. This treatment protocol has been proven effective in providing durable tumor control for most cases. In some instances, palliative courses with several weekly doses of radiation may be an alternative, especially for larger tumors, possibly in combination with chemotherapy.

Side Effects of Radiation Therapy

Radiation therapy for thyroid tumors can cause side effects, such as radiation dermatitis and mucositis, which typically occur around three to four weeks after starting treatment. These side effects are limited to the treatment field, which depends on the extent of the tumor. Veterinarians may prescribe antibiotics, anti-inflammatory medications, and analgesics to manage these reactions. Pets may also experience hair loss or pigmentation changes in the treated area. While there is a low risk of serious late effects (1-5+ years after treatment), most acute radiation effects heal within two to four weeks after completion of therapy.

In conclusion, thyroid cancer in dogs can be effectively diagnosed and treated through a combination of diagnostic procedures, surgical intervention, radiation therapy, and chemotherapy when necessary. With appropriate therapy, the prognosis for dogs with thyroid cancer is generally favorable, and survival times can range from several years to even longer. If you suspect your dog may have thyroid cancer, it is crucial to consult with a veterinarian for a proper diagnosis and treatment plan tailored to your pet’s specific needs.

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