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 disruptors) 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 disruptors, some suspect that the chemicals traditionally used to line pet food cans might be involved (e.g., BPA). 

If your veterinarian has confirmed that your cat’s kidney function is still adequate. When 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 in your cat. Read more 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 has been 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 veterinarians can offer you without it. I prefer that you purchase the canned products if you can because cats on dry diets rarely consume sufficient water to stay fully hydrated. Chronic dehydration might eventually lead to kidney issues as well. (read here) Read more about chronic kidney problems in cats here.

Because hyperthyroidism and kidney disease often occur together, your veterinarian will check your cat’s 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. Cats that are taking thyroid hormone after 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 and cat owners, than to rely on static webpage images on the internet, 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 types 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 amounts of iodine, providing distilled water for your pet might 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. Marine fish generally contain more iodine than fresh water fish. A significant part of fish iodine is in or near the fish’s head, where the majority of its thyroid glands are located, as well as in its skin. (ask me for Rehbein2009) If you are still intent on feeding your cat a high fish diet, feed it only fresh water fish (such as farm-raised tilapia), and then only the fillet portions. High supermarket fish diets can lead to other problems too – such as a thiamine vitamin deficiency because many fish contain thiaminase. (read here) And remember that fish fillets, just like red meat and poultry, do not contain sufficient calcium or vitamins to be fed to your cat as a large proportion of its 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 cannot make exact comparisons using the published tables I have access to 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


Another good source of data for selecting low-iodine meat and common foods was the online Iodine calculator of food ingredients on the Japanese-Table-Of-Food-Composition site, 7th Revised Edition.  That site is no longer available to me. But when it was, I made some notes that they calculated that there were zero µg of iodine in 210 grams of chicken broiler meat (thigh meat, without skin, raw). Sasami broiler chicken meat (raw) also had a zero µg of iodine per 50 grams. Large type breed pork loin, fat, raw had zero µg Iodine per 17 grams. These are micrograms, not milligrams. 

Low iodine meats are rich in B vitamins. But they are too low in vitamin A, vitamin D3 and calcium for cats to thrive. Calcium supplements are readily available. Unfortunately, liver, which is high in both those vitamins, also has significant amounts of iodine. The FDA/ USDA tables list Beef/calf, liver as having 16.4 µg Iodine per 100 grams. Twice the amount listed for ground beef. Whereas the B vitamins are safe, too much of vitamin A or D3 in a cat’s diet is unsafe. If you would like to suggest low iodine vitamin D3 and vitamin A supplements that have preformed well in your hyperthyroid cat, let me know, and I will post them at the bottom of this page. Even hyperthyroid cats need sufficient iodine in their diets to support their metabolic needs. For calcium, I suggest powdered eggshells that you prepare at home. (see here)

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

As I mentioned, many hyperthyroid cats have co-existing kidney issues. But their BUN and creatinine levels can appear better than they really are when laboratory tests are preformed, because of their excessive drinking and urine production – until their hyperthyroidism is controlled. If that is discovered after radioiodine treatment, there is not much your veterinarian can do, other than suggest that your cat’s diet be low in phosphorus.  If that situation is discovered while your cat is on y/d or a home prepared low iodine diet, phosphorus binders could be helpful.   

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

Owner email in black, follow-up emails in purple, my comments in green

On Wed, Jun 14, 2023 at 4:07 
I understand from your article that you have some tables with the iodine content of fresh meat products. My cat didn’t do well on Felimazole and the radioactive iodine therapy has a waiting list of a year. So we are now starting on Hills Y/D but I’d like to give him some alternative fresh options. Thank you so much for making this information available, it is of such help. I will probably use a combination of Hill’s Y/D to make the diet as complete as possible and low Iodine fresh food, since I find my cats thrive on fresh food. Some muscle meat, chicken, turkey, tilapia. I saw that heart is not so bad either. I just started and I am still trying to get them all to eat separate food with Surefeed. That doesn’t work perfectly yet. But slowly getting there. But I’ll give you some first results as soon as I can. I also give Punk (my thyroid cat) some melatonin. This might mitigate the damage from ROS (reactive oxygen species) that a high amount of T4 invokes. It also keeps him a little calmer at night. I use a dose of 0,3 mg per night. Humans usually use 2 – 3 mg. He does quite well with the melatonin. Best regards, H.R, Utrechtseweg, Arnhem

Dear H.R., The 2003 Norwegian analysis you found is much better than the 2006 I had posted. I added it to the article this morning. If you attempt to prepare a diet at home, I would be very interested to know how it affects your cat’s T4 level. Providing sufficient vitamin A & D3 might be a problem. B vitamins are plentiful in meat, but most A & D is found in the liver – and liver has a fairly high amount of iodine. Perhaps a modest amount is OK. Both A & D can cause problems if consumed in excess. Most human vitamins such as Centrum provide carotenoids rather than vitamin A to get around overdose problems. Centrum also has a terrible taste when split, cracked or powdered. It is the large amount of potassium chloride in it that tastes bad. But cats do not, to my knowledge, have a mechanism to convert carotenes into retinol. I have not found any pet vitamins that do not also include iodine. RSH

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