A Home-Cooked Low Iodine Diet For Your Hyperthyroid Cat

Ron Hines DVM PhD

Veterinarians really don’t know why so many older cats nowadays develop hyperthyroidism. Some suspect that cat food contains too much iodine. (read here) Others believe cat food is too low in iodine. (read here  &  here) Still others believe that environmental pollution with compounds that mimic hormones (endocrine disrupters) are to blame. (read here)  I personally suspected that this last theory is at least partially correct because the incidence of hyperthyroidism is increasing in people too. ( read here) That more cats are becoming ill due to the low quality of the fish ingredients in cat foods could also be true. (read here) They show you fish fillets on the can or sac, but in reality it is mostly the non-edible anterior portions of fish ( heads, gill area, and skin) that often make their way into your cat’s food. That plus the fish that sat too long on the dock. Among the endocrine disrupters, some suspect that the  chemicals traditionally used to line pet food cans might be involved (eg BPA). 

If your veterinarian has confirmed that your cat’s kidney function is still adequate. If your finances allow it. If your cat is not yet in its twilight years; then I believe that radioiodine treatment is still your best option in dealing with hyperthyroidism. Read all about hyperthyroidism in your cat here

Until recently, your veterinarian had only three ways to deal with hyperthyroidism: Treat your pet with anti-thyroid medications (methimazole/Tapazol®) or destroy the overactive cells in your cat’s thyroid glands with radioactive iodine or when possible, surgically remove the malfunctioning portions of your cat’s thyroid glands. 

Do I Have A Fourth Option Now?


All three treatment options I mentioned have their benefits and drawbacks. However in the last few years it was found that limiting the amount of iodine in your cat’s diet often allows its thyroid glands to reduce production of thyroid hormones down to normal levels. (read here) The first person I know of to discover that was a New Zealand veterinarian and his associates. (read here)  However those same research scientists were unable to lower thyroid hormone levels long term with their low-iodine diets. Based on that knowledge, Hills Prescription Diets, used the same approach for hyperthyroid cats and it appeared to be successfully – at least over a 3-month period. In another Hills-funded study, their diet appeared to control T4 levels for a full year. (read here) That recipe is now the basis for Hill’s® Prescription Diet® y/d™. It severely limits its iodine content (at or below 0.32 ppm on a dry matter basis). The research of the Hills Company (a subsidiary of Colgate-Palmolive) seems to show that feeding a low-iodine product such as theirs is a fourth option in treating hyperthyroidism in cats. 

It will be wonderful for cat owners and veterinarians if low-iodine diets solve or even help cats with hyperthyroidism problems. If radioiodine treatment is not an option for your cat, I suggest that you either try y/d™ as they suggest, or make their diet a part of your cat’s medical treatment plan. I have never liked the high number of side effects I have sees using Tapazol®/Felimazole® at its recommended dosages. And where I live, few can afford radioiodine therapy for their cats. Even if the addition of the Hills diet is only sufficient in lowering your cat’s required medication doses, I believe that would still be an improvement over what can be offer to you without it. I prefer you use the canned product because cats on dry diets rarely consume sufficient water and many are in the process of developing kidney problems as well. Read about kidney problems in cats.  Because hyperthyroidism and kidney disease often occur together,  your veterinarian will check your cats BUN and creatinine blood levels and probably your cat’s ability to concentrate its urine (urine specific gravity) and perhaps run a microalbuminuria test. Those are the ways we judge the health of your cat’s kidneys. Then your veterinarian will periodically assay your cat’s blood thyroid hormone levels (T4) while on the new diet. Your cat needs to make the transition to this diet gradually. If it is already taking medications to deal with its hyperthyroidism, those medications need to be tapered off gradually. Cat that are taking thyroid hormone subsequent to radioiodine treatments do not have this option. Cats that have stable thyroid hormone levels after radioiodine therapy do not need (and should avoid) a low iodine diet. But cats that have again become hyperthyroid might benefit from y/d® as well. When y/d® is working, your cat should begin to gain back some of its lost weight and, more importantly, its blood T4 level should return to normal or high-normal levels. Signs like vomiting, weight loss, an unkempt hair coat and hyperactivity that may have brought you to your veterinarian in the first place should improve.

Hill recommendations on the use of y/d® change as the Company gains experience with their product. It is always safer to read their current online recommendations to veterinarians than to rely on a static webpage such as this one.  

Must My Cat Eat The Hills Diet For The Rest Of Its Life?

Not necessarily – although It will always need a low-iodine diet.

Once you know that a low-iodine diet is helpful for your cat you have another option. You can prepare a low-iodine diet at home. I suggest that because I think that maintaining your cat on a diet that is primarily corn and soybean-based, will bring on its own set of health issues. (read here)

Hill’s research seemed to show that a diet that had an iodine level at or below 0.32 ppm (=parts per million) on a dry matter basis returned the blood total thyroxine level of cats to the normal range within 8-12 wks. Experimental, non-hyperthyroid cats, fed a diet as low as 0.17 ppm of iodine for a year also seemed to do well.

How Will I Know The Iodine Content Of The Ingredients I Decide To Feed My Cat?

The chief ingredients to avoid are fish and sea products, particularly the type of fish products that often find their way into commercial cat food. (read here). Iodized salt and human foods that contain it, medications that have been flavored with fish additives, supplements and vitamin preparations that have iodine among their ingredients and kelp and seaweed-containing products should be avoided. If your home water contains substantial iodine provide distilled water for your pet could be helpful. Although iodized salt should not be an ingredient in these cat’s diets, adequate salt, in itself, is essential. (read here)

Here are some foods that physicians tell their patients to avoid when trying to limit their iodine intake: iodized salt, sea salt, aged cheeses, egg yolk, sea-products, including kelp, cured meats, bread and pasta that contain iodated dough conditioners or iodized salt, foods and medications that contain dyes (e.g. FD&C red dye #3, erythrosine etc.), molasses, soy sauce, soy milk, meats injected with flavor enhancers, fish oils – including cod liver oil. Dairy meat and egg Iodine levels also vary depending on how much iodine has been added to the livestock’s feed. (read here) If your cat goes outside unattended, whole prey animals that it catches also contain iodine.

The iodine content of fish is quite variable. In general, marine fish contain more iodine than fresh water fish do, and a significant part of that iodine is in or near the fish’s head (where the majority of its thyroid glands generally are) and it skin.( ask me for Rehbein2009) If you are still intent on feeding your cat fish, feed it only fresh water fish (such as farm-raised tilapia) and then only the fillet portions. Supermarket fish can lead to other problems – like thiamine deficiencies (thiaminase-activity). (read here) And remember that fish, just like red meat and poultry, does not contain sufficient calcium or vitamins to be fed to your cat as a sole diet.

Hills expresses the iodine content of their y/d diet in parts per million or ppm of dry ingredients . Most nutritionists publish their data as the quantity of iodine present in 100 grams of prepared (“wet”) food ingredients. Most food ingredients that one would use in preparing a cat’s home cooked diet are 50-80% water. So you can not make exact comparisons using the published tables I know about without doing a bit of mathematics. But you will have a good idea as to which ingredients are likely to be low in iodine and which are likely to be high.

1 microgram (ug, µg or mcg)=0.000001 gram = 1 ppm = 1 ug/gram of food

You may find the Norwegian Food Tables helpful. You can read their iodine analysis of food ingredients. (ask me for NorFdTable2006) Another good source of data for selecting low-iodine meat and common foods is the Japanese-Table-Of-Food-Composition. You can ask me for that one too.  Another extensive analysis of the iodine content of foods was performed in Greece. I have that one, Koutras1970, as well. Be specific when you ask for them as I have thousands of PDFs. As I mentioned earlier, please also remember that you cannot maintain your cat on an all-meat diet without additional calcium and vitamin supplementation.  

What About Keeping My Cat’s Dietary Phosphorus Intake Low?

As I mentioned, many hyperthyroid cats have co-existing kidney problems. But the BUN and creatinine levels of these cats can appear normal when checked due to their excessive drinking – until their hyperthyroidism is controlled. At that time their blood levels of BUN, creatinine and phosphorus can rise to dangerous levels. Cats with overly-high blood phosphorus levels benefit from a diet low in phosphorus as well as a very high fluid intake. (read here) So home cooked diets for these cats need to be moist and low-to-moderate in their phosphorus content as well. That generally means a diet higher in plant ingredients and low-phosphorus grain ingredients than I generally recommend for a meat-eating carnivore like a cat. You can read about the phosphorus content of various food ingredients. ask for USDAphosRef  A second approach is to give these cats phosphorus binders

The whole concept of iodine restriction with a commercial product like y/d® or through a home-cooked, nutritionally balanced, low iodine diet needs to be carefully thought out by your veterinarian. The added stress of a diet change can be too great for some of those cats to bear. If you send me your experiences with y/d® or home cooking, I will post them here. 

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