Dietary Therapy for Hyperthyroidism in Cats

February 24, 2012 (published) | March 24, 2021 (revised)
Photo courtesy of Depositphotos

There always seems to be the occasional cat for which the traditional therapies do not seem to be appropriate.

  • Radiotherapy is not an insignificant expense and may be financially out of reach. 
  • Some cats simply will not take oral medications. 
  • The compounded ear ointments do not always work. 
  • Some owners are simply unable to give medications twice daily indefinitely with meaningful regularity. 
  • Some cats have concurrent illness that preclude methimazole or simply do not tolerate its side effects.

As you may recall from earlier sections, thyroid hormone is made with iodine. It should not be too surprising that the production of excessive amounts of thyroid hormone requires excessive amounts of iodine. Since iodine comes from the diet, it turns out that it is possible to create a diet that is restricted enough in iodine to preclude production of excessive amounts of hormone yet not so restricted that an iodine deficiency results. Hills Pet Nutrition has developed such a diet, called y/d, and it has been an alternative to the more traditional therapies since 2011. The diet is available in both dry and canned formulations.

How Long do you Have to Feed the Diet for the Cat not to be Hyperthyroid Anymore?

Several studies have been done and hundreds of cats evaluated. Many cats will have normal thyroid levels in the first two months of diet use; however, full response can take up to 12 weeks. By 12 weeks on the diet, 90% will show normal thyroid levels. The diet is ineffective in approximately 10% of cats eating it.

Can a Cat Have any Treats While on this Diet?

Unfortunately, there are no acceptable treats. Feeding ANY thing other than the therapeutic diet could interfere with effectiveness of treatment. Foods or treats meant for other pets in the home should be kept away from a cat on this form of therapy. Also, hyperthyroid cats who roam outside may be eating any number of things out in the world. It is unlikely that their diet can be controlled enough for this form of therapy to be effective. In fact, if the cat on y/d is still hyperthyroid after 8 weeks, it can be assumed that the cat is finding another iodine source. The cat might simply be cheating on the diet, getting extra iodine in a medication or in drinking water, or even from the surface of a food bowl.

Can Other Cats in the Household Eat this Diet Safely?

This diet has not been fully evaluated for long-term use in cats that are not hyperthyroid so the manufacturer recommends against its use in normal cats.

If normal cats are in the house and all the cats will have access to this diet, sequester the normal cats daily and feed them regular cat food that is inaccessible to the hyperthyroid cat. Normal cats may safely snack on the diet and the manufacturer gave it to normal cats for one year periods with no apparent adverse effects. It is not known how the y/d diet would affect a normal cat after many years of feeding, though, plus the diet has only been available since 2011 so how cats fare on it over many years remains unknown.

Can a Cat Become HYPOthyroid on this Diet?

None of the hyperthyroid cats tested become hypothyroid after eating this diet. In fact, hyperthyroid cats fed diets vastly more restrictive on iodine than this still did not become hypothyroid.

What Kind of Follow-Up Testing is Appropriate for a Cat on this Diet?

The manufacturer suggests taking a week to transition the cat from his normal food to this one not because of the iodine issue but because it is always a good idea to avoid an abrupt food change. After the transition is complete, the manufacturer recommends a thyroid level, kidney parameters, a recheck exam and a urine specific gravity (test for urine concentration) after 4 weeks and after 8 weeks on the diet. If your cat is still not showing a normal thyroid level in 8 weeks, it is worth going out to 12 weeks and evaluating one more time. After your cat has achieved a normal thyroid level, an exam and blood work should be performed every 6 months.

In cats with concurrent kidney disease, lab work is recommended after 2 weeks, 4 weeks, and 8 weeks on the diet and then every 3 to 4 months thereafter.

What about Switching a Cat on Methimazole Over to the Diet?

The manufacturer recommends simply switching from medication to diet directly with no transitional period. Simply discontinue the medication and start the diet.

Methimazole, surgery, and radiotherapy are well-reviewed effective therapies for feline hyperthyroidism. Where this diet fits in the picture and whether it should replace traditional therapy or be considered an acceptable last resort remains to be seen over time.

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