Using a Low-Iodine Diet (y/d) in Hyperthyroid Cats Allowed Outdoors

Is feeding the low-iodine diet (Hill's y/d) an appropriate or reasonable choice for a hyperthyroid cat that is allowed outside and hunts? 

Will the diet work to control the hyperthyroid state if the cat eats small prey?

My Response: 

No, Hill's y/d would not work to lower the high serum T4 and T3 in a cat in this situation.

All species (cattle, pigs, chickens, birds, mice and other rodents) all contain iodine in their muscle tissue (1,2). Iodine may be ingested in the diet as a supplement, but it's also present in soil, which ends up in the plants ruminants and other animals may eat (3,4). Remember that iodine is an essential nutrient and is needed by all animals (including humans!).

This cat, by eating birds and rodents (not eating an iodine deficient diet, of course), would ingest too much iodine for the y/d to be effective. To lower T4 secretion, the diet has to be very, very low in iodine.

References:

1. Hemken RW. Factors that influence the iodine content of milk and meat: a review. Journal of Animal Science 1979;48:981-985.  
2. Downer JV, Hemken RW, Fox JD, et al. Effect of dietary iodine on tissue iodine content in the bovine. Journal of Animal Science 1981;52:413-417. 
3. Whitehead DC. Studies on iodine in British soils. Journal of Soil Science 1973;24:260-270.  
4. Fuge R, Johnson CC. The geochemistry of iodine — a review. Environmental Geochemistry and Health 1986;8:31-54.