Feline Hyperthyroidism Treated With y/d Diet

Feline Hyperthyroidism has traditionally been treated with oral medication. As of this writing, the most common form of treatment of this disease is by of use the feline-specific medication, Felimazole, which is supplied in a tablet form. Difficulty with administering pills often leads pet owners to request compounding into a liquid or transdermal form.

If your cat has hyperthyroidism, y/d might be the treatment for you.

Surgery can be performed to remove abnormal thyroid glands.
Risks associated with this procedure are described here.

Until now, the most definitive and easiest means of controlling feline hyperthyroidism has been treatment with radioactive iodine, described here. “Easy” because no medication is required after the thyroid glands have been obliterated by treatment. Expense and logistical issues are the most important limitations to this form of treatment.

The “easy” standard has just been changed, with the introduction of Hill’s Prescription Diet y/d.

Through sixteen studies over ten years, Hill’s Pet Nutrition studied how to control feline hyperthyroidism by limiting dietary iodine intake. It is impossible for the body to create thyroid hormone without iodine. The challenge for Hill’s researchers was to find a way to create a diet with a controlled amount of iodine.

Evaluating cat foods marketwide, Hill’s researchers found that they range in iodine content from 0.5% to 9.0%. For normal cats, excess iodine intake presents no problem because unused iodine can easily be eliminated from the body. In the hyperthyroid cat, however, Hill’s has determined that the ideal dietary intake of iodine for these patients is 0.3 parts per million (ppm). More iodine than that and they produce too much thyroid hormone. Less iodine and the patient may become hypothyroid from under-production of thyroid hormone.

Hill’s Pet Nutrition is acutely aware of the need for consistency in dietary control of the disease. To address that, Hill’s is committed to a 24-month production schedule for y/d so that your veterinarian should never run out.

Most of these patients are older and thus may be finicky about their diets. While palatability of all Hill’s foods is guaranteed, that is not an issue with y/d. Compared to Hill’s most palatable feline foods, c/d and t/d, palatability of y/d is even higher.

In a study of 150 cats, effectiveness of hyperthyroidism control was 100%.

If dietary therapy is to maintain control, cats must eat nothing else. Infrequent ingestion of small quantities of other food sources may not cause problems, but overall compliance must be extremely good. If you have other cats in your household with normal thyroid glands, Hill’s current 1-year-old study shows y/d to be safe for all cats. The study is ongoing, and will culminate at a 3-year point.

“My cat is already taking Felimazole for hyperthyroidism. How can I transition her to Prescription Diet y/d?” Your pet’s doctor will lead you through the process. Hill’s has given him recommendations for reducing Felimazole dosage as your cat transitions to y/d therapy.

Cost of y/d is not significantly higher than Felimazole treatment. Adding the cost of medication plus a top-quality diet yields only a slight overall increase in daily expense, about $0.75 per day in either the canned or dry forms.

Renal disease is often a concern in the typical, older hyperthyroid cat. Hill’s has included that consideration in the formulation of y/d. y/d is safe in kidney failure cats to Stage 4. y/d’s formulation is also suitable for
cats with predisposition to urinary tract disease.

No one treatment mode is appropriate for every case. Yet, y/d promises to become the treatment of choice for the majority of hyperthyroid cats.

See you tomorrow, Dr. Randolph.



  1. We have a rescue kitty that is about 2 years old. She has hyperthyroidism and is taking Methimazole 2.5 mg, twice a day. We also give her Hill’s y/d. Is it okay to do both?

    • The bottom line is that thyroid levels need to be checked several times each year. We don’t want them too high, BUT, we don’t want them too low, either. Thanks for reading, Dr. Randolph.

  2. I have a senior cat. She is not taking the Felimazole at all. Today is her first day with me and I am worried that I may have to take her back to the shelter and I don’t want to. If I take her to my vet and he prescribes Y/D Thyroid cat food, will she be OK to live on. Your article states that she will be. Thank you.

    • Yes, it’s a complete diet and satisfies all of her nutritional needs. HOWEVER, be aware that if she eats ANYTHING with iodine in it the T4 maty climb again. Have you not tried transdermal methimazole? Except for the gooiness on the pinnae, it has no drawbacks. I’ve never seen a patient or an owner have trouble with it. Ask your veterinarian to order it for you. Thanks for reading, Dr. Randolph.

  3. My 17.5 year-old cat has been on the y/d since February and loves it. My questions is: how long can I expect my cat to live with controlled hyperthyroidism? I understand that the diet does not cure the disease, just slows the progression. I would like to know by how much (if it’s even possible to know such things!). Thank you in advance.

    • Unfortunately, I don’t have the answers to your questions, but I’m sure the data compiled by Hill’s Pet Nutrition can answer them, simply by calling the toll-free number on the bag of food. Of course, there are innumerable variables, especially extent of kidney disease as hyperthyroid patients experience damage to their kidneys prior to being diagnosed, then age-related contraction. The extent of kidney disease is often masked in the unregulated or poorly-regulated hyperthyroid patient. Thank you for these challenging questions!

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