Bruno’s Dog Blog Issue XLI – “My Hyperthyroid Sister, Sox the Cat.”

I know that it has been a while since I last wrote, but things have been a bit hectic for us recently. Unfortunately, we had a death in our family and the holidays are already a busy time of year anyway. It’s a new year and there is no shortage of topics to discuss!

As many of you may already be aware, February is National Pet Dental Health Month. I always like to take this time of year to raise awareness about the importance of oral hygiene in our pets and strive to encourage all pet owners to brush their pet’s teeth on a daily basis, like my Mom does for me! Every February I write about dental health in our pets, so this year, I am reposting an old blog issue to refresh your memory on the topic along with a video showing how my Mom brushes my teeth every night! Please look for the “Bruno’s Dog Blog Issue XXXIV – “Smile, it’s dental health month!” For something new, I am going to take some time to tell you a story about my friend, Sox the cat, and her journey through the diagnosis and treatment of hyperthyroidism.

Hyperthyroidism is a common disease of middle-aged and older cats that is caused by an increase in production of thyroid hormones from an enlarged thyroid gland in the cat’s neck. In most cases, enlargement of the thyroid glands is caused by a non-cancerous tumor called an adenoma. However, in rare cases, hyperthyroidism can be caused by a malignant tumor, known as thyroid adenocarcinoma. At this time, the cause of feline hyperthyroidism is not known.

Cats afflicted with hyperthyroidism usually develop a variety of signs that may be subtle at first, but that become more severe as the disease progresses. The most common clinical signs of hyperthyroidism are weight loss, increased appetite, and increased thirst and urination. Hyperthyroidism may also cause vomiting, diarrhea, and hyperactivity. The coat of affected cats may appear unkempt, matted, or greasy.

Thyroid hormones affect nearly all of the organs in the body; therefore, thyroid disease often causes secondary problems, including heart disease and high blood pressure (also known as hypertension). However, once the underlying hyperthyroidism has been controlled, the cardiac changes and hypertension will often improve or may even resolve completely. Additionally, once treated, hyperthyroidism may lead to the unmasking of underlying kidney disease in some cats, which may require additional management.

Diagnosing hyperthyroidism in a cat involves performing a thorough physical examination with palpation of the cat’s neck area to check for an enlarged thyroid gland. A blood chemistry panel and an analysis of thyroid hormone levels along with a CBC (complete blood count) and urinalysis should be performed to evaluate the overall internal organ health and function. Most cats with hyperthyroidism will have elevated levels of the thyroid hormone T4 in their bloodstream, but a small percentage of cats with hyperthyroidism can have T4 levels within the normal range. In these cases, additional tests may be recommended to obtain the diagnosis of hyperthyroidism. Since hyperthyroidism can predispose a cat to other conditions, it is important to evaluate their general health, with particular focus on the heart and kidneys.

There are four treatment options for feline hyperthyroidism: medication, radioactive iodine therapy, surgery, and dietary therapy. Each treatment option has its advantages and disadvantages. The treatment a cat receives for hyperthyroidism will depend on specific circumstances, including the patient’s overall health status, the owner’s ability and willingness to medicate the cat regularly, and financial considerations.

The anti-thyroid drug, called methimazole, acts by reducing the production and release of thyroid hormone from the thyroid gland. This medication does not provide a cure for the disease, but it does allow either short-term or long-term control of hyperthyroidism. The advantages of medication are that the drug is readily available and relatively inexpensive. Some cats may experience side effects from medication, however, including vomiting, anorexia, fever, low red blood cell and/or white blood cell counts, lethargy, and facial pruritus. Lifelong treatment, usually involving twice-daily oral dosage, will be required to control the disease. Methimazole is also available in a gel that can be applied to the skin for transdermal absorption. Regardless of the formulation of the medication, blood tests are required periodically during treatment to evaluate whether the therapy is effective by measuring the circulating T4 blood level, to monitor kidney function, and for potential side effects.

Radioactive iodine therapy is considered the treatment of choice for cats with hyperthyroidism. During treatment, radioactive iodine is administered as an injection and is quickly absorbed into the bloodstream. The iodine, which is required for the production of both T3 and T4 hormones, is taken up by the thyroid gland and the emitted radiation destroys the abnormal thyroid tissue without damaging the surrounding normal tissues. The majority of cats treated with radioactive iodine have normal hormone levels within one to two weeks of treatment.

The advantages of radioactive iodine therapy are that the procedure most often cures hyperthyroidism, has no serious side effects, and does not require anesthesia. It does, however, involve the handling and injection of a radioactive substance that is only permitted at facilities specially licensed to use radioisotopes. The radioactivity carries no significant risk for the cat, but precautionary protective measures are required for people who come into close contact with the cat during and after treatment. Radioiodine is safe and effective with cure rates approximately 95 – 98% with one treatment.

Removal of the thyroid glands, called surgical thyroidectomy, is a relatively straightforward surgical procedure that has a good success rate. The advantage of surgery is that it is likely to produce a long-term or permanent cure in most cats, and therefore eliminates the need for long-term medication.

Finally, limiting the amount of iodine in the diet may be a viable option for treating hyperthyroidism in some cats. This may be particularly useful in cats with medical conditions that make other treatment options impossible or in those cats who experience adverse effects with medication. The prescription diet, Hills Y/D is the only iodine restricted diet available for treatment of hyperthyroidism in cats. Typically with dietary therapy, hyperthyroidism can be controlled within 3 months. However, it is extremely important that the cat is fed this diet exclusively. Even what may seem to be an insignificant amount of extra human food or treats fed to the cat that is not the Y/D diet can prevent this method of treatment from working to control the disease.

The prognosis for cats with hyperthyroidism is generally good with appropriate therapy. In some cases, complications involving other organs may worsen this prognosis.

Now that you have an understanding about what hyperthyroidism is and how it is treated, I can tell you about what happened to Sox. Over the summer, Mom noticed that Sox was drinking and urinating a lot more than usual. She also had a ravenous appetite, but that wasn’t unusual for Sox! Mom diagnosed Sox with hyperthyroidism by testing her blood as described above and feeling an enlarged thyroid gland in her neck.

Mom initially started Sox on the medication, methimazole; however, within 3 weeks of starting the medication, Sox became very ill. She stopped eating and developed a fever. Mom brought her into work for another check-up and performed some repeat blood work. She found that Sox has a dangerously low white blood cell count, which was preventing her body from being able to fight off infection from normal bacteria in her body. Sox required hospitalization for intravenous fluid therapy, antibiotics, and supportive care, while her body was working hard to rebuild its normal supply of white blood cells. The good news is that with withdrawal of the methimazole medication and some supportive care, Sox was able to recuperate and her white blood cell count returned back to normal.

Since Sox experienced a rare adverse effect from the methimazole, medication was no longer a viable treatment option for her hyperthyroidism. Sox unfortunately, also has other underlying diseases, including inflammatory bowel disease, chronic pancreatitis, and chronic hepatitis, that prevented Mom from being able to use the dietary therapy with the prescription diet Y/D as a treatment option for Sox. This left only surgery or radioactive iodine for treatment of Sox’s disease, so Mom opted for the radioactive iodine therapy, since Sox had previously experienced an adverse event while undergoing anesthesia for a dental procedure last year, making Mom less than excited about anesthetizing Sox again in the future.

It took some time before Sox could get an appointment for the radioactive iodine treatment. She had to go through some additional screening to evaluate her heart function, blood pressure, and look at her internal organs with an ultrasound, but she eventually was cleared for the procedure. However, there was a delay in obtaining the radioactive isotope required for her treatment. Sox was scheduled and subsequently canceled for her treatment on 4 separate occasions before the radioactive iodine was finally able to be obtained. She went in for her treatment on Monday, January 29 and returned home on Friday, February 2, 2018.

Hospitalization is required under the radioisotope permit issued by New York State for 5 days following injection with he radioactive iodine. Sox, and other cats that have been treated with radioiodine have a low level of radioactivity following discharge from the hospital. This level is sufficiently low that complete isolation of the cat from people is not required. However, there are some guidelines that we do have to follow for 3 weeks after Sox comes home to eliminate or further reduce human exposure to radiation that is emitted by Sox and by radioiodine that is excreted in her urine and saliva. These precautions include not allowing Sox to sit on our laps or lay on our head/neck area for prolonged periods and she should not be allowed to sleep with us during this time. Additionally, we have to collect all of her waste from the litter box and hold onto it until March 2, 2018, before disposing of it in the trash like we normally would.

It was weird not having the cat home with us for the week that she was gone for her treatment, but I am happy to say that she is back with us once again! I was so excited to see her that when she got home, I knocked over her carrier because I was sniffing it so vigorously! I could hardly contain my excitement to see my friend once again! I do feel bad that she is locked out of the bedroom at night since she can’t sleep with us for the next few weeks, but she doesn’t seem to mind. I think the hardest part is keeping her off of Mom’s lap at night while we are all relaxing and watching TV together. She just desperately wants to crawl up on her chest and sit millimeters from her face, but she can’t, at least not for the next few weeks.

It’s a small price to pay knowing that she will be cured of her disease once and for all! I hope that you found this entry educational and informative in case you have a feline friend who may also be hyperthyroid. If you think your cat may be hyperthyroid, just know that it is a treatable disease with a good prognosis, and call your veterinarian to get your feline friend checked out!

Until next time, your pal,


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