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Older Cats and Hyperthyroidism     

Dr. Al Townshend

Hyperthyroidism is caused by an increase in the production of thyroid hormones. It is seen in about 10% of cats ten years or older. The thyroid gland is located in the neck and is difficult to see normally. Feline Hyperthyroidism produces an enlargement.


What Causes Hyperthyroidism?

In almost all cases, the cause is a benign tumor called an adenoma. Rarely a cancerous tumor termed an adenocarcinoma is the cause. They both create a lump that can be easily felt and seen.

Bisphenol A (commonly referred to as BPA), a coating found on the inside of some canned cat foods, has been thought to play a role in causing Feline Hyperthyroidism. Many manufacturers have eliminated their use of BPA. A 2020 study1 found no link between BPA and increased thyroid hormone.


What are the Signs?

Because thyroid hormones directly affect most of the body’s organs, the signs can vary, especially if there are pre-existing problems before thyroid issues develop. Initially, the signs can be very subtle, but as the disease progresses, they can become much more severe.

  • Weight loss even though the cat is eating normally or their appetite has increased.
  • Increased water intake and urination.
  • Most appear unkept.
  • Apprehension and increased activity. 
  • Vomiting and diarrhea can also occur.

Other conditions can cause some of the same signs, so getting the kitty to the veterinarian is essential for an early and quick resolution.



The Guardian’s detailed description of the signs, a physical exam to detect any thyroid gland enlargement, and blood and urine tests are generally required. Occasionally additional testing may be necessary.

Blood and urine tests can also detect any other conditions the cat may have that can complicate the treatment options and require follow-up testing after the thyroid issues are resolved.

Without treatment, the condition will progress and directly affect the pet’s quality of life and lifespan.


Treatment Options

Depending on the cat’s general condition, there are four treatment options.  The cost of each treatment option can also be a factor the Guardian must consider.



Anti-thyroid drugs reduce the production and release of thyroid hormone. It is essential to understand that these medications do not cure the condition but can effectively control the symptoms. The medications are efficient and inexpensive compared to other treatment options. 

 Follow-up testing to assure the proper blood levels are achieved is critical in effectively controlling the condition.



Surgery requires general anesthesia, which can increase the risk in older cats. The procedure usually involves the removal of part of the thyroid gland. Care must be taken not to damage nerves, blood vessels, and the parathyroid gland, which lies close to the thyroid gland.

If all the diseased tissue is removed, surgery, in most cases, produces a cure for the condition.


Radioactive Iodine Therapy

The procedure involves injecting radioactive iodine, which is taken up by the thyroid gland to make thyroid hormone. It destroys the tumor cells without damaging the normal cells. Cats return to normal production of thyroid hormone within two weeks.

Cats undergoing treatment need to be hospitalized during treatment and for 3-5 days after until radiation levels fall to acceptable limits. Most cats recover completely with no side effects. 

A disadvantage is that there are only a limited number of facilities capable of performing the procedure, so travel is usually involved.


Dietary Restriction

Limiting the amount of iodine is another method of controlling Feline Hypothyroidism. All commercial pet foods are designed to meet the pet’s nutritional needs, including adequate iodine. There are no foods with lower than required iodine except Hill’s Prescription Diet y/d canned and dry recipes. 

The advantage is that it is a low initial cost option. Cats that are difficult to medicate, at risk of surgery, or where radioactive therapy is unavailable may only have this option. 

There are concerns with this form of treatment that should be considered:

  • Hypothyroid cats must eat only this food and nothing else. Care must be taken not to let the pet eat the other cat’s food.
  • Multiple cat families must ensure the other cats in the home do not eat this food. It is too low in iodine and can cause problems.
  • The food is not as palatable as other foods
  • Many Guardians object to the list of primary ingredients; Corn Gluten Meal, Whole Grain Corn, Chicken Fat, Powdered Cellulose, Egg Product, Chicken Liver Flavor…
  • Low Iodine can complicate other disease conditions the cat might have.

All treatment options should be discussed with the veterinarian to ensure the best choices for the pet and the Guardian.



  • The veterinarian should closely monitor cats to be sure thyroid levels are normal and stable. 
  • Hyperthyroidism can have a significant effect on the heart and kidneys. Once thyroid levels are achieved, other organs may become a problem and need therapy.



The therapy options to resolve or control Feline Hyperthyroidism all have a good prognosis for most pets. The sooner the pet is diagnosed and treated, the better the prognosis.



Older cats should all be supported with optimum nutrition and supplements that encourage a long and happy life.

  • Nutrition is always a key factor. The feline species are true or obligate carnivores, requiring meat in their diet. Highly digestible, animal protein-focused and low carbohydrate recipes are ideal. 
  • Pre- and probiotics and digestive enzymes support an aging and less efficient digestive tract. Probiotics are an essential part of the immune system,
  • Omega 3 fatty acid supplements such as fish oils, hemp seed oil, and phytoplankton support whole body health and are essential for heart and kidney health and skin and hair health.
  • Older cats, like older dogs, often suffer from the discomfort of arthritis. Glucosamine, chondroitin, and MSM supplement can support joint health and relieve some of the discomforts.


Additional Resources

  1. Source Kovařiková S, Maršálek P, Habánová M, Konvalinová J. Serum concentration of bisphenol A in elderly cats and its association with clinicopathological findings. J Feline Med Surg. 2020;23(2):105-114.

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