However, soy will increase the profit margin for pet food companies

There are a couple likely environmental factors that have led to this epidemic. Firstly, multiple studies have been done that have pointed to certain cat foods as risk factors for developing hyperthyroidism. The food offenders included fish for their high levels of polybrominated diphenyl ethers (PBDEs), which are a known endocrine and thyroid disruptor commonly found in fire retardant furniture, carpet and dust. Also referenced in the studies were known endocrine disruptors Bisphenol A (BPA) which is used to line some cans of cat food, and soy, which is a prevalent ingredient in many types of cat food.
Humans are more than aware that soy, BPA and the PBDEs found in high concentrations fish are hormonal disruptors in our own systems. Some of these chemicals may be banned from human consumption in the United States, but are still widely used and imported in pet food. Even the FDA warns us, especially women of child bearing age and children, against consuming these same ingredients included in cat food.
Another potential contributing factor to feline hyperthyroidism is iodine levels in food. Iodine is necessary for the thyroid to work properly, yet the levels of iodine in cat food widely vary. Iodine is highly prevalent in seafood, and too much iodine could result in an overactive thyroid. The pet food industry has acknowledged the correlation between iodine levels and hyperthyroidism in cats and many manufacturers have introduced low iodine prescription diets.

Jan 3, 2012 - Pet food companies are now introducing 'low-iodine' formulas for hyperthyroid cats

On the other hand, we do not know anything about the bioavailability of iodine in cat foods. Cat foods very high in iodine are possibly enriched with iodine-containing pigments for food (e.g., erythrosine). Normally, the digestibility of these pigments is very low and iodine is thus only insignificantly bioavailable.

What should I feed my hyperthyroid cat

cats is affected by brief administration of food high or low in iodine content There are a couple likely environmental factors that have led to this epidemic. Firstly, multiple studies have been done that have pointed to certain cat foods as risk factors for developing hyperthyroidism. The food offenders included fish for their high levels of polybrominated diphenyl ethers (PBDEs), which are a known endocrine and thyroid disruptor commonly found in fire retardant furniture, carpet and dust. Also referenced in the studies were known endocrine disruptors Bisphenol A (BPA) which is used to line some cans of cat food, and soy, which is a prevalent ingredient in many types of cat food.
Humans are more than aware that soy, BPA and the PBDEs found in high concentrations fish are hormonal disruptors in our own systems. Some of these chemicals may be banned from human consumption in the United States, but are still widely used and imported in pet food. Even the FDA warns us, especially women of child bearing age and children, against consuming these same ingredients included in cat food.
Another potential contributing factor to feline hyperthyroidism is iodine levels in food. Iodine is necessary for the thyroid to work properly, yet the levels of iodine in cat food widely vary. Iodine is highly prevalent in seafood, and too much iodine could result in an overactive thyroid. The pet food industry has acknowledged the correlation between iodine levels and hyperthyroidism in cats and many manufacturers have introduced low iodine prescription diets.

Managing Hyperthyroidism With Food

Many cats with hyperthyroidism develop diabetes if their condition isn't controlled, which can complicate matters. Even if your cat doesn't officially have diabetes yet, his hyperthyroidism may trigger the beginning stages of insulin resistance. Foods too high in carbohydrates can push him from not-quite diabetes into full-blown diabetes without warning. Many cat foods also contain iodine, which is what the thyroid gland uses to create thyroid hormones. Too much iodine essentially keeps supplying the thyroid gland with the building materials necessary to keep overproducing.

Research Suggests Low Iodine Diets are Safe for Healthy Cats | petMD