What is Thyroid Cancer?
Hyperthyroidism can cause tumors to form on the endocrine glands in the neck. These are noticeable lumps that you can feel upon rubbing the neck of an affected cat. The tumors are generally one of two types of cancer. Benign adenomas are the most common type of cancer related to hyperthyroidism. They are referred to as benign because this type does not metastasize (spread) to other body parts. Malignant adenocarcinomas are cancerous tumors that are aggressive and do spread throughout the body. Of the two types, benign adenomas are easier to treat. Either form of cancer tends to go untreated for quite some time, but the earlier it is caught and treated by a veterinary professional, the better. Failure to treat the condition can result in organ failure and death.
Feline thyroid cancer is a gradually developing cancer that is sometimes seen in older cats (usually above the age of twelve). It can take months or years to manifest. The thyroid consists of two lobes of endocrine glands in the upper neck. It is responsible for the production and secretion of the hormones thyroxine (T4) and triiodothyronine (T3), which regulate the pace of the metabolism and other body functions. When the thyroid starts to produce too many of these hormones, hyperthyroidism develops in the cat.
Symptoms of Thyroid Cancer in Cats
The most common sign of thyroid cancer is a sudden, consistent surge in energy. While this may seem like a good thing, it can be the sign of an overactive thyroid. Monitor this, and watch for other symptoms, such as:
- Polyphagia (increased appetite)
- Polydipsia (increased thirst)
- Polyuria (frequent urination)
- Urination outside of litter box
- Weight loss
- Change in behavior
- Increase in energy
- Racing heart beat
- Heart murmur
- High blood pressure
- Dull fur
- Hair loss
- Incessant meowing
- Blindness (in very advanced cases)
Causes of Thyroid Cancer in Cats
When the thyroid begins to secrete excessive amounts of thyroid hormones, hyperthyroidism, and in turn, thyroid cancer develops. The exact cause is not fully understood, however, it is believed that a series of events and circumstances lead to cancer. Possible causes include:
- Diet consisting of too much iodine
- Environmental chemicals
- Radiation exposure
Diagnosis of Thyroid Cancer in Cats
If you or your veterinarian suspect thyroid cancer, an appointment should be made to investigate the cat’s condition. A full medical history will be needed to see if the cat has an extensive history of hyperthyroidism. A physical exam will be completed, with focus on the under chin and neck to feel for nodules under the skin. These can be noticeable lumps or chains of tissue. The vet will also take care to differentiate between thyroid cancer and diabetes, as they do share some symptoms.
Blood work will be needed, including a complete blood count and a biochemical profile. These tests will help see how many blood cells are present. T4 serum levels will be checked to see if they are registering higher than 10 times the normal amount. Another indication of cancer is if the cat has no response to either methimazole or carbimazole. The amount of thyroid hormones present in the body will be tested. Urinalysis may be done to exam the function of the kidneys and liver.
X-rays can show if the tumors present are irregular or multifocal, and can be used to monitor any spreading. Often, a scintigraphy will be requested, as it is useful for further assessing tumors and determining appropriate dosages for treatment. The cat will need to be kept at the clinic for 2-3 days after this test, as it uses radioactivity that stays present in the cat for a time. Cardiac ultrasound may be needed if heartbeat irregularities are found. If the cat’s condition is stable enough, an excisional biopsy (full removal of the tumor) will be performed. When microscopically evaluated, this can lead to a definite confirmation of cancer present.
Treatment of Thyroid Cancer in Cats
A few different treatments are available depending on the type of cancer present and how far it has progressed in the cat. Often, a combination of these treatments will be recommended for best results.
Radioactive iodine has been found to be very effective in treating thyroid cancers. It is non-invasive and no general anesthetic is needed. A dosage of radioiodine is injected under the cat’s skin. It will treat all tissue that is hyper functioning in the body, and kill all abnormal cells. The cat will need to stay at the clinic or hospital for up to four days to eliminate all radioactive materials to be considered safe at home. A higher dose is given for cats with adenocarcinomas. This treatment has a 94% success rate.
Surgical Debulking or Removal
This treatment is often combined with radioiodine administration. At the time of biopsy, all tumors greater than 3-4cm in diameter will be removed. If congestive heart failure is occurring due to thyroid cancer, a full removal of the thyroid glands may be needed. This requires general anesthesia and is a higher risk surgery. Surgical complications are common.
In very progressed cases of thyroid cancer, medication to slow progression may be the only option. Antithyroid medication can reduce rapid onset of thyroid issues and lengthen life expectancy. These medications will need to continue for the duration of the cat’s life and can be expensive.
Recovery of Thyroid Cancer in Cats
If the cat responds positively to radioiodine treatment, the general prognosis is very good. Extended survival lengths have been noted even in advanced cases of thyroid cancer. It takes 3-4 weeks to verify whether the treatment is fully successful or not. If it is, there is only a 0.3% chance of remission.
If the cat has undergone surgery, check frequently that the incision is clean and dry. The vet may prescribe calcium or vitamin D supplements for the healing process. In all treatments, the vet will monitor T4 levels in 4-6 weeks, and then periodically after that. Recurrence does happen, especially when radioiodine has not been administered. If the cat is not a good surgical candidate due to advanced cancer, a very restrictive diet may be prescribed to improve and lengthen lifespan.
Thyroid Cancer Questions and Advice from Veterinary Professionals
To my knowledge my cat, thankfully, doesn't have cancer but I do have a question for a veterninary professional that I would like answered if at all possible. Please see below.
My cat, about 12 years old, is being treated for hyperthyroid, the medicine being used is Mehimazole tablets 5mg. At first she was given 1 tablet am and 1 tablet pm, when her blood work was done to determine her levels later - her thryoid function was now too low so the dosage was change. New dosage was 1/2 tablet am and 1/2 tablet pm she stayed on this medication about 3 months. She recently was rechecked and her thyroid level had come up some but it was still too low. The doctor first told me to increase her dosage to 1 tablet am and 1 tablet pm when I questioned this he changed it to 1 tablet am and 1/2 tablet pm then he thought about it and changed it to 1/2 tablet am (said he got the cat and dog treatment mixed up) He finally told me the dosage was to be dropped down to 1/2 tablet am only. I am really confussed - is my cat on the correct dosage or should I thinki about changing to another doctor? Any and all suggestions and advice will be greatly appreciated.
Please email me at [email protected]
Methimazole is used to reduce the level of thyroid hormone in the body, the higher the dosage the more suppression of hormone production; meaning that if a cat is being treated with methimazole and the thyroid hormone level is too low, a reduction in dosage will (in theory) increase the level of circulating thyroid hormone. It is possible that your Veterinarian made a genuine mistake, as we are all humans after all. There is no real “correct dosage” as a trial and error “sweet spot” needs to be found to keep the thyroid hormone within physiological range, which is common in these types of conditions. Regular monitoring will show if you are on the right path. Regards Dr Callum Turner DVM
If a cat has thyroid cancer would the levels initially respond to methimazole? My cat has been on it for 4 months, levels came back to normal. We caught it early but he is not gaining any weight back which is concerning to me.
Thanks so much to Dr. Callum Turner for taking the time to answer my questions about Susie. His reply help to make me feel better about the situation. THANK YOU, Dr. Turner.
My 16 year old cat was treated with Methimazole instead of the radioactive treatment for his thyroid several years ago,was declared "cured" after about a month of treatment, and has been on 5 mg twice daily ever since--the vet said that he would need to take it the rest of his life. We have not tested him periodically to check this as suggested here. Now he has cancerous cells in his thyroid, and they've basically given me no hope for him, as he is also diabetic, and wouldn't survive surgery. If I were to increase the dose of the methimazole, would that be a treatment that could address the cancer cells? I'm really in need of a 2nd opinion. Thank you.
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