Q & A: Anorexia in a Hyperthyroid Cat on Hill's y/d

I have a 12-year old female DSH cat with hyperthyroidism that we have been managing with the new Hill's y/d diet. The cat was eating both the canned and dry y/d, and the serum T4 fell from 7.0 μg/dl to 3.2 μg/dl after 2 months.

So everything was going along fine on y/d without any other medication. But now the cat has completely stopped eating for the last 2 days. Other than the cat's palpable thyroid tumor and low body condition score, my physical examination was normal. Repeat routine blood work did not reveal any abnormalities.

So what should we do in this situation? The owner suspects it's an issue with the taste of the y/d and thinks that she would eat her "regular" cat food.

Is there something else to do first before doing a further workup searching for a concurrent disease? If we give her another food, will she become hyperthyroid once again?

My Response:

This is one of the major disadvantages we have when relying on a extremely low iodine diet (Hill's y/d) to control a cat's hyperthyroidism.  Let's face it — Hill's y/d isn't the tastiest cat food on the market, and most cats (if not all) would prefer to eat another type of diet.

But more importantly is the fact that almost all cats, as they age, will develop episodes of partial to complete anorexia secondary to nonthyroidal illness. In these sick cats, we often first tempt them to eat with different foods (fish-flavored, etc) or use recovery-type diets before embarking on a major workup for a concurrent disease. However, feeding these cats any other food but y/d increases their iodine intake, and will result in relapse of the cat's hyperthyroid state.

How long to relapse on the y/d diet is stopped? Well, that depends on a number of factors including the following:

• The size of the hyperthyroid cat's thyroid tumor (which will continue to grow on the y/d diet).
• The magnitude of the cat's original serum T4 value (i.e., higher T4 values reflect an increase in severity of hyperthyroidism, with a larger thyroid tumor size).
• The time on the y/d diet.
• How low the cat's T4 fell on the y/d diet, which reflects how iodine depleted the cat's thyroid tumor became on the diet. In other words, if the serum T4 decreased into low-normal range of on the y/d, we would expect the cat to remain euthyroid longer than if the T4 only fell to upper limit of the reference range, as in this cat.

In any case, we know that the serum T4 will increase again, once the y/d is not fed exclusively. You must continue this iodine deficient diet for the rest of the cat's life for this diet to continue to manage the hyperthyroidism. So we would want to get them back on y/d and off the "temporary" foods as soon as possible. If they remain on a diet other than y/d for any length of time (certainly longer than 1 to 2 weeks), it's likely that they will show relapse and T4 will have to be monitored.

Remember that once they relapse, it's going to take a few weeks feeding the y/d diet exclusively for the serum T4 to again fall into the euthyroid range.

Bottom Line:

Veterinarians have to be prepared for the fact that many, if not all, senior hyperthyroid cats will develop concurrent diseases as they age, some of which will require management with a diet other than Hill''s y/d. This is one of the major downsides of using this diet management for the long-term control of cats with hyperthyroidism.

Owners must be made aware of the fact that another treatment method might be needed if the cat's disease state dictates the need for another diet plan.

Link to My Other Posts on the Hill's y/d Diet