First described in 1979, feline hyperthyroidism has become an increasingly common endocrinopathy of geriatric cats, and concentration of iodine in commercial foods may be a contributing factor.

Multiple cat-food brands (112 over-the-counter and prescription diets) from 3 geographic regions (ie, west coast, Florida, Midwest) were purchased and iodine concentration measured. Seventy-one were canned, 19 were pouched, and 22 were dry foods. Eight brands and 2 supermarket varieties were tested. The daily iodine intake for a hypothetical 4.5-kg cat or 1.4-kg kitten was calculated based on manufacturer feeding instructions. Although no significant difference was found in iodine concentration across the 3 regions, variations were noted across packaging types, brands, seafood ingredients, and intended use (ie, therapeutic or not). Canned foods showed the greatest variation, in which an intake between 49 and 9,639 µg iodine/day was calculated.

Diagnosis of feline hyperthyroidism has become a near daily event in my practice. Many owners, especially those with their second or third affected cat, question if they can do something to prevent unaffected cats from developing an active thyroid nodule. Avoiding known thyroid disrupters (eg, flame retardants, fish-based diets with high levels of polybrominated diphenyl ethers [PBDEs], soy, bisphenol-A lined cans) can be difficult, if not impossible. Because fluctuations in iodine content of food (from low to high levels) have been linked to toxic nodule goiter development in humans and cats alike, why not simply feed a diet with consistently low iodine? The only commercially available iodine-restricted diet is, in this commentator’s opinion, too low in protein and too high in carbohydrates to meet an obligate carnivore’s long-term needs.

Until manufacturers produce cat foods with consistent iodine levels (after first determining what levels cats actually need), or at least report levels on the nutrient content statement, my clinic will be well stocked with methimazole.—Sandra Sawchuk, DVM, MS

Iodine concentration in commercial cat foods from three regions of the USA, 2008-2009. Edinboro CH, Pearce EN, Pino S, Braverman LE. J FELINE MED SURG 15:717-724, 2013.