Thyroid Cancer in Cats: Symptoms and Common Causes

Most thyroid tumors in cats are benign, not cancerous. 

However, a small percentage of cats develop thyroid cancer, a malignant (cancerous) tumor that can spread to other parts of the body and may require treatment such as radioactive iodine therapy, surgery, radiation, or chemotherapy. 

Thyroid cancer is most often diagnosed in senior cats. 

While thyroid cancer is uncommon compared to benign thyroid disease, early diagnosis and treatment can help improve both quality of life and long-term outcomes. 

What Is Thyroid Cancer in Cats?

Thyroid cancer is diagnosed when a tumor found on the thyroid gland is malignant. This means that  it can spread throughout the body. 

The thyroid consists of two small, paired glands located in the neck near the cat’s throat. The thyroid glands are responsible for regulating metabolism. 

Nearly 98% of tumors found in the thyroid gland are benign adenomas. While these tumors usually lead to hyperthyroidism, or excessive thyroid hormone production, they do not spread and are not considered cancerous. The remaining tumors are usually diagnosed as thyroid carcinomas. 

Cats with thyroid tumors may develop a lump on the neck that can sometimes be felt during an exam. These masses are often so small that they are difficult to detect without veterinary experience. The tumor begins producing too much thyroid hormone, and affected cats develop side effects related to the hormone overproduction. 

Thyroid cancer is generally not considered a medical emergency unless a cat experiences a thyroid storm. A thyroid storm is a sudden, severe increase in thyroid hormone levels. These cats have a racing heart, elevated body temperature, weakness, and respiratory distress. 

If you notice any of these symptoms, seek veterinary care immediately. 

Symptoms of Thyroid Cancer in Cats

Cats with thyroid cancer often develop signs related to excessive thyroid hormone production. Symptoms may include: 

• Weight loss 

• Vomiting 

• Agitation or irritability 

• Increased appetite 

• Increased vocalization or meowing 

• Increased drinking and urinating 

• Diarrhea 

• Poor grooming 

• Lump on the neck 

Causes of Thyroid Cancer in Cats

Thyroid cancer in cats doesn’t have a single known cause. However, as with many other cancers, both genetics and environmental factors are suspected to play a role. 

While no specific breeds are known to be at higher risk, thyroid cancer is more common in senior cats. 

Diets that are excessively high or low in iodine may increase a cat’s risk for thyroid tumors. 

Exposure to environmental toxins, such as BPA, may also increase a cat’s risk of thyroid carcinoma. 

How Veterinarians Diagnose Thyroid Cancer in Cats

Because both benign and cancerous thyroid tumors can lead to hyperthyroidism in cats, differentiating between the two can be tricky. 

Your veterinarian will start with a thorough physical exam. They will palpate the neck and determine if the thyroid glands are enlarged. Many cats with hyperthyroidism also have a heart murmur, in addition to weight loss and the symptoms listed above. 

Blood work, including thyroid hormone testing, is used to screen for hyperthyroidism. If your cat’s total T4 level is elevated, this supports a diagnosis of a thyroid mass. 

To determine whether the mass is benign or cancerous, veterinarians may recommend additional testing, including biopsy and scintigraphy. 

A biopsy involves surgically collecting a tissue sample from the thyroid gland for laboratory analysis. Scintigraphy involves injecting a radioactive isotope that helps differentiate cancerous tissue from non-cancerous tissue. 

Some pet parents may instead pursue a treatment trial. 

This involves starting treatment for hyperthyroidism and monitoring the cat’s response. Cats with benign thyroid disease often respond well to traditional hyperthyroidism treatment, while cats with thyroid carcinoma may not. 

Treatment of Thyroid Cancer in Cats

Thyroid cancer is often treated with radioactive iodine therapy (I-131). The thyroid gland naturally absorbs iodine, so veterinarians use a radioactive iodine isotope to selectively destroy the tissue. 

Because radioactive material is used during treatment, cats must stay at a specialized treatment facility for several days to weeks to prevent radiation exposure to family members while radiation levels decrease to safe levels. 

Once treatment is complete and radiation exposure is no longer a concern, cats are allowed to return home. 

Surgery is another treatment option for thyroid cancer. The affected thyroid gland or glands may be surgically removed. 

Following surgery, the tissue is submitted for histopathology (biopsy) to confirm whether all cancerous tissue was successfully removed. 

Some pet parents elect traditional radiation therapy rather than radioactive iodine treatment. This involves multiple radiation sessions directed at the thyroid gland. 

Chemotherapy may also be recommended in some cases. While many chemotherapy medications are injectable and require outpatient hospital visits, Palladia is an oral chemotherapeutic medication that may be recommended. 

Rarely, cats may receive a low-dose NSAID, such as Metacam, as part of palliative care to improve comfort and quality of life. 

Recovery and Management of Thyroid Cancer in Cats

If your cat is treated with I-131 therapy, your veterinarian may recommend limiting prolonged close contact for about two weeks after your cat returns home. 

Some pet parents find it helpful to use a large dog crate that comfortably fits a litter box, bed, and food and water bowls during this period. 

If your cat undergoes surgery, a soft recovery cone may help prevent scratching or rubbing at the incision site while it heals. 

Recovery from surgery generally takes about 10-14 days. 

Cats receiving chemotherapy, alone or in combination with an NSAID, may benefit from probiotic therapy to support healthy gut flora while on medication. 

Both Purina FortiFlora and Nutramax Proviable are commonly recommended options. 

Prevention of Thyroid Cancer in Cats

Because genetics likely play a role in thyroid cancer, prevention can be difficult. 

However, pet parents can try to minimize environmental factors that may contribute to excessive thyroid stimulation and tumor development in cats. 

Helpful preventive steps may include: 

• Avoiding plastics when possible. Use stainless steel, glass, and ceramic food and water bowls instead. Use foil food pouches rather than plastic-lined tins. 

• Limiting excessive fish-based canned diets, which may contribute to excess iodine intake. 

• Avoiding diets excessively high in soy or deficient in selenium. 

• Filtering your cat’s drinking water to reduce exposure to environmental contaminants and pesticides.