Hill's y/d for Feline Hyperthyroidism - Managing Hyperthyroidism in Cats with Diet

Hyperthyroidism is the most common glandular disorder in cats.  It is mainly seen in elderly cats, usually over 10 years of age.  It occurs when the thyroid gland in the neck starts producing too much thyroid hormone.  This causes cats’ metabolism to increase, and cats will start burning calories like crazy, causing them to lose weight.  They try to compensate by eating more food, but they usually cannot keep up, and cats will lose weight despite having an excellent (often ravenous) appetite.   Other clinical signs are possible, such as excessive thirst and urination, vomiting, diarrhea, panting, restlessness or hyperactivity, and excessive vocalization at night.

(See: Hyperthyroidism in Cats - The Fact Sheet)

Traditional treatment options include radioactive iodine treatment, surgical removal of the thyroid gland (rarely done anymore), and the administration of anti-thyroid drugs.  Recently, the Hill’s company has introduced on the market a new diet, called Hill’s Prescription Diet y/d Feline Thyroid Health.  The diet is very iodine restricted.  Iodine is necessary for the synthesis of thyroid hormone.  By depriving the cat of iodine, the thyroid cannot produce excessive amounts of thyroid hormone, and the condition comes under control.

The ideal treatment for hyperthyroidism is radioactive iodine.  One injection of radioactive iodine usually cures the problem.  The disadvantage of this treatment is the cost (it’s pricey), and the fact that the cat needs to stay at the facility for approximately 7 to 10 days until the levels of radioactivity in the urine and feces are acceptable.   If clients are unable or unwilling to pursue radioactive iodine therapy, anti-thyroid medication is the next most common treatment.  The drug methimazole (brand names Tapazole  and Felimazole), when administered twice daily, will bring the thyroid level under control.  An advantage of oral medication is that it is less expensive.  Disadvantages are the need to give the drug twice daily.  Cats can be difficult to medicate.  For cats who fight taking a pill, the medication can be compounded into a liquid formulation.  For cats who refuse to have ANYTHING put into their mouth, the medication can be formulated into a gel that is smeared onto the hairless, inside part of the ear.  It will be absorbed through the skin, and it will lower the level of thyroid hormone.  Another disadvantage is the potential for side effects.  In some cats, methimazole will suppress the bone marrow, causing either a low white count and/or low platelet count.  In some cases, methimazole will cause intense facial itchiness, and cats will scratch frantically, often really tearing up their faces.   In these cats, the only other option is radioactive iodine.  (Surgery is still an option, but no one pursues that anymore due to it being the same cost as radioactive iodine, and the risks of anesthesia.) If a client whose cat cannot tolerate methimazole cannot pursue radioactive iodine due to financial restrictions, we’d be in trouble.  Fortunately, we now have an additional option: diet.

Whether your hyperthyroid cat is a good candidate or not for the Hill’s y/d diet depends on many factors.  Cats that have an illness that is already being nicely controlled on diet (for example, diabetes, bladder stones, or inflammatory bowel disease) probably should not be switched to Hill’s y/d, since there are other options for hyperthyroid treatment.   Cats that are young when they are diagnosed with hyperthyroidism (for example, 10 or 11 years old) should strongly be urged to pursue radioactive iodine therapy rather than food or medicine. Cats are living longer than ever.  If oral medication is chosen, we’re looking at the possibility of giving oral meds for maybe 7  or 8 years.  You could feed Hill’s y/d for those 7 or 8 years, but the odds are that the cat will grow tired of the same food for this long a period of time.  So, “young” hyperthyroid cats should have the radioactive iodine therapy.

The diet works by severely limiting the amount of dietary iodine intake.  By restricting the iodine, you deprive the cat of one of the essential building blocks for production of thyroid hormone.  The only real caveat:  the cat is allowed to eat Hill’s y/d, and ONLY Hill’s y/d.  Absolutely NOTHING else can be eaten – no treats, and no people food.  The iodine level in non-y/d food is enough to nullify the effects of the y/d.  Most medications that are given to cats won’t interfere with the effectiveness of Hill’s y/d.  For example, Dasuquin (an arthritis medication), Epakitin (a kidney supplement), Miralax (a constipation remedy), etc. can be given to cats receiving Hill’s y/d.  However, Azodyl, Heartgard Chewables for Cats, Feline Greenies Pill Pockets, Felovite, Nutrical, and Enisyl lysine paste and lysine treats COULD interfere with the effectiveness of Hill’s y/d and should be avoided.  Contact the Hill’s company if your cat is receiving a supplement and you want to know if it might interfere with the diet.  

Most cats find the food to be palatable.  If the cat doesn’t like the food, or starts to show disinterest, you cannot mix any other type of food with the y/d to make it more enticing.   Hill’s y/d comes as a canned and a dry formulation.  The Hill’s company has provided veterinarians  with literature that contains recipes for making “snack triangles” from the canned formula, and “snack cookies” from the dry formula, so cats that are really used to getting fed treats can be given these homemade treats made out of Hill’s y/d.  Call Hill’s, or ask  your vet for details.

The marketing of Hill’s y/d now gives veterinarians another option for treating hyperthyroidism.  If, after speaking to your veterinarian about this diet, you still have questions, cat owners can call Hill’s Veterinary Consultation Service at 1-800-548-8387.  They will answer all of your questions.