Another friend of ours just had a cat diagnosed with hyperthyroidism, and I wanted to share an overview of the things I shared with her. Having had four hyperthyroid cats in our family, this is something I’m familiar with.

What is hyperthyroidism?

Hyperthyroidism is a hormone disorder in cats, where the thyroid gland produces too much of the thyroid hormone. Cats with hyperthyroidism often lose weight, seem restless or even hyperactive. Some hyperthyroid cats can even seem aggressive.

What you can’t see from the outside is that a hyperthyroid cat’s body has to work a lot harder with all these extra thyroid hormones telling it to go, go, go. That makes the condition hard on a cat’s heart and kidneys.

The thing your vet often doesn’t say is that all hyperthyroid cats have a tumor, usually benign, in their thyroid causing it to overproduce. I had four hyperthyroid cats, including one that went through radioactive iodine treatment, and no one vet said the word tumor to me! This experience doesn’t seem unusual, and as far as I can tell, some vets leave that detail out to keep explanations simple.

What causes hyperthyroidism?

This is a question under much debate. In humans, hyperthyroidism is often caused by the autoimmune disorder Grave’s Disease or by iodine insufficiency. It’s not clear what causes it in cats. It is possible that flame retardants in our furniture and sofas may contribute to this condition in cats. There is also a theory that PBDEs and polychlorinated biphenyls (PCBs), found in the fish ingredients in cat foods, may be to blame.

How is feline hyperthyroidism treated?

There are four ways you can treat a hyperthyroid cat:

• Anti-thyroid medication
• Radioactive iodine (I-131) treatment
• Surgical thyroidectomy
• Prescription food

Anti-thyroid medication

The most common form of treatment is an anti-thyroid drug. This prevents the peroxidase enzyme and iodine in your cat’s system from being used by the thyroid gland to create as much thyroid hormone. Since this treatment reduces the amount of thyroid hormone based on the amount of the drug taken, there is often a period of trial and error to get it adjusted so that it does its job without going too far and making the cat hypothyroid, which can cause other health issues.

Daily pill box with cats printed on it. Photo credit: flickr creative commons/Lisa Risager

This treatment also does not address the underlying issue that there is a tumor in the thyroid gland which will continue to grow. Over time, the thyroid gland can become significantly enlarged, or the tumor itself can change from benign to cancerous.

Radioactive iodine (I-131) treatment

Today, treatment with radioactive iodine (I-131) is considered the treatment of choice for hyperthyroidism in cats. Since iodine is required for the thyroid gland to function correctly and generate thyroid hormones, a radioactive form of iodine that can destroy thyroid cells is used to treat the overactive thyroid. While the thyroid gland stores iodine, no other parts of the cat do, so the treatment is localized and focused to the problem gland. The tumor is destroyed, and the cat is cured of hyperthyroidism.

This kind of treatment is not invasive, but it is performed on an inpatient basis at a specialty veterinarian because of the need for the facility to have special licenses to handle the nuclear material. They generally keep a cat from 3 to 7 days while most of the radiation is emitted from the cat’s body.

My cat, Rhett, had radioactive iodine treatment for his hyperthyroidism.

I-131 treatment isn’t perfect. A small percentage of cats require a second round of treatment when the first isn’t fully effective. Additionally, despite the dose of radioactive iodine used for treatment being less than it was five years ago, there is a possibility of a cat becoming hypothyroid and being unable to create enough thyroid hormone on his own. Many cats who are hypothyroid look great and don’t show any signs of low thyroid. If they do, your vet will advise supplementing with a hormone replacement.

Surgical thyroidectomy

Before I-131 treatment was available, the removal of the thyroid gland by surgery was a treatment employed by veterinarians. Usually, both lobes of the thyroid gland are removed, since otherwise the disease usually recurs within a few years.

This is not considered the favored course of treatment, because it is a long surgery, and patients with hyperthyroidism are often senior or geriatric cats who often also have issues with their heart or kidneys that make extended anesthesia risky. Because this treatment removes both lobes of the thyroid, cats must be supplemented with thyroid hormone for the rest of their lives, which is not ideal.

This treatment is no longer in very much in the US, and some veterinary schools have even stopped teaching students how to perform it.

Prescription food

A newer form of managing hyperthyroidism is through a cat’s diet. Prescription anti-thryoid diets are available that don’t contain iodine. Without iodine, the thyroid doesn’t have all of the necessary ingredients to generate the thyroid hormone, so it can’t overproduce it.

Cat kibble. Photo credit: flickr creative commons/rfduck

This food is controversial, since it does nothing to treat or remove the tumor causing the hyperthyroidism. In order to remove the iodine, most of the protein is removed, as well, and much of the protein is provided from non-animal sources. Although the thyroid overproduction is controlled, muscle wasting continues because when the cat’s diet is too low in protein, the body starts to consume its own muscles for an energy source.

Another problem with this approach is a practical one. Cats don’t like this food. In a cat food manufacturer study, 225 cats began the trial, and after only two weeks, only 60 cats remained because the others rejected the food. Yet to work, a cat has to eat this food and only this food, with no treats or other snacks to prevent any iodine entering the cat’s diet from other food sources. This kind of discipline is hard to make possible, especially in a multi-cat home.

Every cat is different

What’s right for your hyperthyroid cat may be different than mine. What was right for one of my cats wasn’t the same for the next of my hyperthyroid cats. Your vet will guide you through the pros and cons of different treatment options.

The main thing to remember is that as scary as it is to hear that your cat has hyperthyroidism, there are lots of options for helping your cat feel better.

Research and further reading:

Journal of Feline Medicine and Surgery, Hyperthyroidism in cats: what’s causing this epidemic of thyroid disease and can we prevent it?
Environmental Science & Technology, Higher PBDE Serum Concentrations May Be Associated with Feline Hyperthyroidism in Swedish Cats
NAVC Conference 2017, Hyperthyroid Diagnosis and Treatment
American Thyroid Association, Radioactive Iodine
Insights into Veterinary Endocrinology, The Best Diet to Feed Hyperthyroid Cats

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