Yes. Fish oil contains a large amount of trans fats per volume due to the high heat refining process and the long shelf-time at high temperatures. It's mainly oxidized PUFA. Omega 3 in general slows thyroid, let alone rancid omega 3. I'd estimate your dog became hypothyroid from the supplementation.
Originally Posted by magnolia1973
In addition, carbohydrate blocks free fatty acid oxidation, so I would estimate a pure carnivore would suffer much more from fish oil supplementation than a human. Our bodies convert carbohydrate into SFA and MUFA when in excess, which counteracts the PUFA. Since dogs don't eat carbohydrate and can't do this, I would imagine toxic fats would affect them much more quickly.
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