Published by: Ingrid King. Last Updated on: June 26, 2023 by Crystal Uys

A cat eating food

For the past few months, the veterinary community has been bombarded with ads for a new feline prescription food that is said to cure hyperthyroidism in 3 weeks.

When the diet first came out, I was skeptical. I’m not a fan of prescription diets. While I respect the research that goes into these diets, sadly, they usually contain ingredients such as meat by-products, corn, soy and grains, none of which are optimal for an obligate carnivore like the cat.

Hyperthyroidism is a common disease that typically affects middle-aged and older cats.  It is caused by an excess production of thyroid hormones, which are produced by the thyroid gland, located inside the cat’s neck. Thyroid hormones affect nearly all organs, which is why thyroid disease can sometimes cause secondary problems such as hypertension, heart and kidney disease.

There are currently three treatment options:  lifelong medication, surgery, and the gold standard, radioactive iodine therapy. A single injection of Radioiodine (I-131) cures 98-99% of feline hyperthyroidism cases without any adverse side effects. There aren’t many diseases that have that simple a cure and such a high cure rate.

Hill’s Pet Nutrition’s new diet, y/d Thyroid Health, is supposed to provide a fourth option for pet owners. My gut feeling was that it sounded too good to be true: simply change your cat’s food, and cure a potentially life-threatening disease? The ingredient list made me cringe. The dry version contains no animal protein; its protein is derived from corn gluten meal, soybean hulls and dried egg product. The canned product is only marginally better: it contains meat by-products, corn and rice.


Having dealt with a hyperthyroid cat, and understanding the challenges of either giving twice daily medication for the rest of the cat’s life, or facing the considerable expense of the radioactive iodine treatment, I would love it if a cure could be a simple as changing a cat’s diet. However, after doing some research, I’m not convinced that it’s a simple as that, even if a cat guardian would be willing to compromise on good nutrition and feed a diet with what I consider questionable ingredients.

Dr. Jean Hofve, a retired holistic veterinarian, wrote in her Little Big Cat newsletter:

“While Hill’s admits that there is no scientific evidence that excess iodine causes hyperthyroidism in cats (in fact, their staff veterinarian stated that if iodine were actually the problem, every cat would be hyperthyroid!), they nevertheless claim that this diet, fed exclusively, will normalize a cat’s thyroid levels in 3 weeks. The diet has been tested for nutritional adequacy by feeding trial in approximately 14 cats; some cats ate the food for 1-2 years.

Time will tell if y/d will be a “cure-all” for feline hyperthyroidism; and if it’s truly safe for long-term use. The claims being made by Hill’s seem pretty outrageous for a generally conservative company. Expect these claims to be vigorously disputed by many.”

cat eating food from bowl
Image Credit: Seattle Cat Photo, Shutterstock

Dr. Eric Barchas, veterinary contributor to Catster, remains skeptical as well. In his article Can a New Thyroid Health Food Live Up to the Hype?, Barchas states:

“So far, I have tracked down only two studies on y/d. Both consisted of small samples run over relatively short time periods, and both were run by Hill’s. Both showed that the diet is effective, but anyone with a shred of scientific background should be able to tell that the studies aren’t very strong, and that more research is indicated.

Larger, long-term studies are needed to determine what happens when cats without thyroid disease eat y/d. Plenty of people have more than one cat, and cats eat from one another’s bowls. Also, a very significant number of cats with hyperthyroidism also have kidney disease. How will this diet affect kidney function? Finally, most people like to plan on their cats living for several more years (I would hope that a 7-year-old cat diagnosed with hyperthyroidism might make it to 15 or even 20). I believe the longest study on record so far lasted two years. What will happen to cats that eat y/d for 10 years?

In short, I believe that Hill’s has unleashed y/d with too little research and too much hype.”

Amber became hyperthyroid in 2006, and I chose the radioactive iodine treatment for her. If I were faced with a diagnosis of hyperthyroidism for one of my cats again, I would most likely make that same choice again. I definitely would not choose this diet.

Featured Image Credit:, Shutterstock

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