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Treating the hyperthyroid cat. Is a restricted diet enough?

Posted January 23, 2012 @ 4:21pm | by Steven R. Shadwick, DVM, Board-Eligible in Internal Medicine

The release of Hill’s new Feline Thyroid Health diet (y/d) has stimulated a lot of conversation recently. Many are interested in the potential a simple diet has in the treatment for the one of the most common endocrinopathies we see in veterinary medicine today. For sure, controlling weight loss, polyuria, hypertension, excessive grooming, night-time howling and frantic behavior by simply changing the diet is an attractive alternative to conventional therapies, but can we expect good results with this new product?

Hill’s Nutrition reports that this new diet is effective at reducing serum thyroid hormone concentrations and is safe to feed on a long-term basis to cats affected by hyperthyroidism. These statements are based on three abstracts recently published in veterinary journals. In these studies, iodine restricted diets have been shown to reduce serum T4 concentrations when fed as the sole source of nutrition to hyperthyroid cats. At low enough iodine concentrations in the food, positive control cats will achieve euthyroid status in about 3 weeks. During the treatment period, no adverse effects were reported in the treated cats in the studies.

While trying to keep an open mind about this new treatment option, there are a few initial concerns about treating hyperthyroid cats with an iodine-restricted diet. I think the most obvious concern people have expressed about this diet is simply, “Will it work?” We have some abstracts that show these diets work, but a research cat in a stainless steel cage is much different than your clients’ pets who live in real-world situations. In order for this diet to offer any benefit, affected cats must be willing to eat the food, must not get any table scraps, and must not get into any food provided for other pets in the household. As you know, this is no small feat. Food trials for food allergies contain the same restrictions; as a reasonable comparison to iodine restricted diet, one can easily imagine how difficult it will be for clients to comply with this treatment protocol.

A possible greater concern than the potential failure of owner compliance is the nutritional content of this new food. It is well known that cats require higher levels of protein in their diets as they age. This is even more important for cats with hyperthyroidism. Hill’s y/d is a relatively low-protein diet. If fed based on Hill’s feeding recommendations on their website, y/d will only provide 4.0 – 4.5 g of protein/kg/day to cats eating this diet. Some reports and expert opinion place this at 50 – 75% of the protein requirement for older cats or cats with hyperthyroidism. Compounding the effects of limited protein content is the protein quality of this diet. Corn gluten meal, the primary protein source in y/d, is considered a relatively poor-quality protein source. Knowing that hyperthyroidism is a catabolic process, this poor-quality protein provided in limited amounts is likely (perhaps guaranteed) to contribute to the muscle wasting noted in older cats and cats with hyperthyroidism.

Based on limited clinical information, high potential for poor compliance, and the potential for exacerbating the muscle wasting and weight loss that affects hyperthyroid cats, I do not recommend this diet as the treatment of choice for feline hyperthyroidism. Like all therapies for any disease, one should consider how the entire animal is affected and not simply a number on a page. There may be specific cases where this diet can be prescribed, but in general it should be after traditional therapies have failed or are unavailable or inappropriate for specific reasons.

If you have specific questions or comments about this diet or hyperthyroidism in general, please feel free to come see me at the Minnesota Veterinary Medical Association (MVMA) Conference on February 2-4, 2011. I will be at the Animal Emergency and Referral Center booth #407, or contact me anytime at AERC.

Steven R. Shadwick DVM
Residency-Trained in Internal Medicine
Animal Emergency and Referral Center of Minnesota
651-501-3766

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