Fish Oil and Hyperthyroidism — Is There a Link?

November 18, 2010
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By Jean Hofve, DVM

A reader recently quoted part of one of our newsletters in an email, and (in addition to the subject of her email, which was our warning about alpha lipoic acid) expressed concern about Omega-3 fatty acids and their potential association with high iodine levels and feline thyroid disease. She was wary of fish oils, and although I can see how her concern came about, the basis of many of her worries is unfounded. I’d like to set the record straight.

Iodine is stored in the body in the thyroid gland. Products that use fish, fish meal, or fish oils that are derived from processing or pressing the whole fish (including the thyroid) will contain that iodine. Cod liver oil is pressed from fish livers after they are removed from the fish, and is not a significant source of iodine. The best fish and cod liver oils are also molecularly distilled, which further removes virtually all solids and other impurities. The purest Omega-3 fish oils contain, at most, a tiny trace of iodine. Good quality, fresh Omega-3 fish and cod liver oils (such as pharmaceutical-grade products from Nordic Naturals) are not the problem when it comes to iodine.

Most of the iodine consumed by our pet cats comes from commercial cat foods, primarily due to the mineral pre-mixes used in these foods. Minerals, including iodine, are often included in great excess. Iodine is difficult and expensive to test for, so it’s cheaper and easier for pet food makers and suppliers to add way more than necessary to make sure the food is not deficient. They don’t care if too much gets into cat food. One study of adult and growth dog and cat foods found 10-100 times the recommended amount for many minerals. There are few maximum limits set (12 for dog food, 4 for cat food). Iodine has a maximum limit for dog food, but no similar maximum in cat food. This approach makes the food cheap, but it can cause a host of physical problems for our cats.

In addition, you may recall that a study out of Purdue University found an association between certain flavors and types of cat food and hyperthyroidism pointed out that pop-top cans (most of which contain BPA in their linings), giblets (by-products), and fish had the highest associations. Again, the fish going into these products are whole feeder fish, damaged or spoiled fish, or fish parts (heads, tails, internal organs) not wanted for human consumption. Iodine levels in fish meal made from these products is likely to be quite high. Fish meal is a common ingredient in cat food and on many labels is credited as the source of Omega-3 fatty acids. In reality, it is a very minor source of Omega 3’s and is used in cat food mainly for flavoring. Fish oil derived from similar sources could be similarly contaminated, which is why we stress choosing a distilled, pharmaceutical grade fish oil for your cat (I personally use and recommend Nordic Naturals).

Further studies have also suggested a link between fire-retardant chemicals and feline hyperthyroidism. It turns out that cheap pet food ingredients–particularly fish and by-products–are the most likely to be contaminated with these chemicals. This could not only account for the Purdue findings, but it could also be creating a “double-whammy” for our cats in terms of exposure to these toxins.

We hope this clarifies the issue and reassures our readers that pure, properly processed fish and cod liver oils contain (at most) trace levels or zero iodine, and certainly are not implicated in any situation of iodine overdose. The fact remains that Omega-3’s are deficient in virtually every pet food, raw food, and even homemade food for pets, simply because of the way livestock are raised in North America. Omega-3 fatty acids are an essential component of your cat’s diet, and must be supplemented for optimal levels to be reached.

Click here to read more about feline hyperthyroidism and its suspected causes.

Click here to read our our original article on Omega-3 fatty acids and their importance for cats.

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2 Responses to Fish Oil and Hyperthyroidism — Is There a Link?

  1. Sharon on April 4, 2011 at 9:51 am

    I would like to start giving my 14-year-old cat fish oil, because I think it might help her with constipation (she sometimes has symptoms of mega-colon). She also is hyperthyroid, too skinny (6 lbs), and her kidneys are shrunken and “pre-renal.” I want to be able to give her a teaspoon of human-grade fish oil but all I can find has added lemon flavor. Will the lemon flavor hurt her? Is it okay to give her a spoonful, and might it help with constipation?

    • jhofve77 on April 4, 2011 at 11:36 am

      Hi Sharon, it is unlikely that fish oil will be of much help with constipation, since it is digested and absorbed before it reaches the colon. (Did you see the article on kitty constipation?) But there are many good reasons to give fish oil beyond that; please see this article: Omega 3s are Essential for Your Cat!

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