Taking newer research into consideration — research that increasingly links Hyperthyroidism to iodine deficiency and draws direct comparisons to goitre in humans — homemade diets, and raw diets in general, may not provide sufficient iodine unless this trace element is purposefully supplemented. Ingredients commonly used in homemade cat food are naturally low in iodine, even when liver and egg yolks are included.

Natural iodine sources such as seaweeds and seafoods can also be unreliable, because iodine accumulation in marine organisms varies considerably depending on environmental conditions. For this reason, standardized iodine sources — such as iodized table salt for homemade diets and pharmaceutical-grade iodine supplements for commercially prepared raw foods — are generally the most consistent and dependable approach.

Why current iodine recommendations may not tell the whole story

At present, iodine recommendations for cats have not been clearly re-evaluated in light of this emerging research. Part of the difficulty lies in how nutrient requirements are traditionally established. In nutritional science, purified diets are created with a target nutrient removed. The resulting health consequences are then observed and corrected by gradually restoring the missing nutrient until health is normalized again.

However, if chronic iodine deficiency contributes to hyperthyroidism, the problem becomes far more difficult to study. Hyperthyroidism may take a decade or longer to develop, making long-term deficiency states extremely challenging to evaluate scientifically. The pet food industry has also been understandably cautious about increasing iodine supplementation without clear guidance, because excessive iodine intake itself was previously suspected as a possible cause of hyperthyroidism in cats.

The timing of the Hyperthyroidism epidemic

One observation stands out historically.

Hyperthyroidism emerged as a major disease in companion cats during the late 1970s — roughly a decade after industrialized pet foods became widely established as the sole source of nourishment for companion cats, effectively divorcing them from what they had done for thousands of years in human company: hunt their own dinner.

Since then, a wide variety of possible causes have been proposed, including fish-based diets, BPA in the lining of cans of canned foods, environmental contaminants such as PBDEs (flame retardants) in house dust and flea-control pesticides, or combinations of multiple contributing factors. Yet no single theory has fully explained the sudden and widespread emergence of this disease.

What we observed in our own study group

During 30 years of observing the study cats at Feline Future Cat Food Company throughout their lifetimes, hyperthyroidism developed between 2015 and 2020 in three cats aged 15, 16, and 18 years old.

These cats had never consumed fish-based diets or canned foods as staples, and had no exposure to pesticides. The homemade raw food they consumed throughout life was also low in iodine to meet official guidelines, suggesting that excessive iodine intake was unlikely to have played a role. Their diet relied on naturally occurring iodine from ingredients such as egg yolk, whey isolate, liver, kelp, and later krill.

For us, this became an important and difficult observation to ignore.

Despite avoiding many of the factors commonly suspected of contributing to hyperthyroidism, the disease still emerged in 3 out of 12 study cats. This suggested that homemade diets and commercial pet foods may share something important in common despite their obvious differences: both remove cats from the nutritional environment they evolved under while regularly hunting and consuming whole prey.

What prey analysis revealed

As part of our ongoing investigation into the nutritional composition of a cat’s natural prey, we analysed wild-caught mice. One finding immediately stood out. The iodine content of these mice was surprisingly high — approximately 3-8 times higher than recommendations currently made by authorities such as the Association of American Feed Control Officials (AAFCO) and the National Research Council (NRC).

Biologically,cats consuming whole prey as a staple diet would naturally ingest the prey animal’s thyroid gland and other tissues where iodine accumulates. Their iodine intake would be quite high.

Studying whether hyperthyroidism occurs in wild cats consuming natural prey diets is difficult, because this disease is primarily seen in senior companion cats that outlive the normal life expectancy of wild felines. We do not know if wild cats simply do not live enough for the disease to manifest, or if they are at a low risk due to mitigating factors – like consuming a prey diet naturally high in iodine. The only meaningful comparison available to us is the companion cat population before and after the rise of commercial diets as sole sustenance. Here, the veterinary community observed a marked shift, with hyperthyroidism suddenly presenting on an endemic scale during the late 1970s — approximately a decade after cats largely stopped hunting for their own food.

How this changed our formulation philosophy

Observing hyperthyroidism in our own study cats caused Feline Future Cat Food Company to lean more heavily into the concept of a prey-model diet regarding iodine supplementation. The goal became to move iodine levels in food prepared with TCfeline closer to what cats would naturally consume through prey.

At the same time, it became increasingly clear to us that, within our own study group, a diet low in iodine, devoid of fish, and not based on canned foods did not prevent this disease.

Managing Hyperthyroidism with diet

Although raw muscle meats are naturally very low in iodine, cat parents with a hyperthyroid cat in their care should not attempt to prepare iodine-deficient diets at home as a means of controlling the disease, or attempt to replicate prescription diets such as Hill’s Thyroid Care y/d Prescription Diet. Hyperthyroidism is a serious medical condition that requires professional veterinary care and supervision. An iodine-deficient diet should certainly never be used in the hope of preventing hyperthyroidism.

Feeding Hyperthyroid cats with TCfeline

Homemade cat food prepared with TCfeline premix is safe and suitable as the staple diet for cats with hyperthyroidism receiving appropriate antithyroid medication such as Methimazole under veterinary supervision. This became well established within our own study group, where cats lived comfortably with medically controlled hyperthyroidism for another three to four years, reaching 19 and even 20 years of age.

Despite the comparatively high iodine levels in the food, these cats required only low doses of oral Methimazole to maintain a euthyroid state, monitored through regular blood testing. The adjusted dietary iodine levels in the diet to match those of prey may have even affected a halt in the progression of the disease.