Hyperthyroidism in Cats

What is hyperthyroidism?

Hyperthyroidism in cats is caused by an increase in production of thyroid hormones (T3 and T4) due to an enlarged thyroid gland. Hyperthyroidism mostly affects cats middle-aged and older, with the average age of affected cats being 12 years old. Among older cats, the prevalence of hyperthyroidism is around 12%.

Possible factors contributing to the development of hyperthyroidism include deficiencies or excesses of certain compounds in the diet and chronic exposure to thyroid-disrupting chemicals in cats’ food or environment. Additionally, the breeds Siamese, Burmese, Persian, Abyssian, and Himalayan seem to carry a higher risk of hyperthyroidism.

What are symptoms of hyperthyroidism in cats?

Symptoms of hyperthyroidism in cats can be hard to identify. Some symptoms can include:

• Weight loss
• Increased appetite
• Increased thirst and urination
• Vomiting or diarrhea
• Hyperactivity
• Unkempt look

These symptoms will typically become more severe as a cat’s hyperthyroidism progresses.

Because the thyroid has effects on the metabolic system, including the heart and kidneys, hyperthyroidism can cause increased metabolic rate, manifesting as elevated heart rate, heart contraction, and blood pressure.

How is hyperthyroidism diagnosed?

To diagnose hyperthyroidism, a vet will check your cat’s heart rate and blood pressure, as well as order lab work and analyze your cat’s thyroid levels. Your vet may choose to do other tests like a urinalysis, chest X-ray, and electrocardiogram (EKG) to establish what the best treatment option for your cat is.

What are treatment options for hyperthyroidism in cats?

Options for treating hyperthyroidism in cats include medication, radioactive iodine, surgery, and nutritional management.

Medication can successfully manage hyperthyroidism in cats, but is not a curative therapy; cats will have to take medication for life to maintain normal thyroid status. Because there are treatment options that can cure hyperthyroidism, medication is typically used for cats that have surgery risks or are very old. The medication used most often to treat hyperthyroidism is methimazole, which is available in both oral and transdermal gel forms. Both of these forms of methimazole can be compounded to be most palatable to your pet, as discussed below. Potential side effects of methimazole therapy can include vomiting, lethargy, anorexia, fever, and anemia. Methimazole can also rarely, but potentially, cause low white blood cells and platelets, so your cat will likely need regular lab work done while taking this medication.

Radioactive iodine is a curative therapy for hyperthyroidism in cats. This therapy works by using iodine, which is required to produce the thyroid hormones T3 and T4, to be taken up into the thyroid gland and emit radiation that kills the abnormal thyroid tissue. Because this therapy involves radiation, it can only be done at special facilities that can handle radioactive substances. As such, cats will need to be hospitalized for three to five days after treatment until radiation levels return to normal.

Surgical removal of the thyroid is also a permanent cure for hyperthyroidism in cats. Because surgery is invasive and requires general anesthesia, it may not be the ideal choice for older cats with other health problems. To normalize thyroid levels prior to surgery, vets may prescribe methimazole for cats to take for several weeks prior to surgery.

Finally, dietary therapy can be useful in some cats to manage hyperthyroidism as a lifelong therapy. This is done by limiting the amount of iodine in the cat’s diet by using food that is low in iodine; there are several cat food brands that offer low-iodine cat food. Iodine restrictive diets, however, are controversial due to the unknown long-term consequences of a low-iodine diet.

Compounded Methimazole

Oral or transdermal methimazole are two compounded medication options for management of feline hyperthyroidism. Oral methimazole can be given to cats in a traditional tablet form, or in palatable oral paste or treat forms which can be compounded to the exact dose needed for your cat. Transdermal methimazole can be supplied as a topical suspension that is applied to your cat’s inner ear, which some cat owners may find to be easier than giving oral medicine. Several studies have shown that transdermal methimazole was safe and effective at treating hyperthyroidism in cats, and can potentially reduce stomach upset side effects that come with traditional oral methimazole treatment. It is important to note that the person applying methimazole should take care to not touch the methimazole solution, as it can be absorbed through the skin and cause low thyroid levels in humans. A discussion about the route of methimazole that is right for your cat should be had with your vet, and compounded methimazole options can be found at https://www.cfspharmacy.pharmacy/methimazole-transdermal-topi-click-micro-compounded.

References

1. Hyperthyroidism in Cats. Cornell Feline Health Center. Updated January 2017. Access at: https://www.vet.cornell.edu/departments-centers-and-institutes/cornell-feline-health-center/health-information/feline-health-topics/hyperthyroidism-cats
2. Köhler I, Ballhausen BD, Stockhaus C, Hartmann K, Wehner A. Prevalence of and risk factors for feline hyperthyroidism among a clinic population in Southern Germany. Tierarztl Prax Ausg K Kleintiere Heimtiere. 2016 Jun 16;44(3):149-57. English, German.
3. Williams K, Downing R, Ward E. Hyperthyroidism in Cats. VCA Hospitals. Access at: https://vcahospitals.com/know-your-pet/hyperthyroidism-in-cats
4. Hoffmann G, Marks SL, Taboada J, Hosgood GL, Wolfsheimer KJ. Transdermal methimazole treatment in cats with hyperthyroidism. J Feline Med Surg. 2003 Apr;5(2):77-82.
5. Vaske HH, Schermerhorn T, Armbrust L, Grauer GF. Diagnosis and management of feline hyperthyroidism: current perspectives. Vet Med (Auckl). 2014;5:85-96.
6. Weir M, Ward E. Transdermal Medications: How They Work and How to Apply Them. VCA Hospitals. Access at: https://vcahospitals.com/know-your-pet/transdermal-medications-how-they-work-and-how-to-apply-them