In theory, an iodine deficient diet starves the thyroid tumor of iodine so that it cannot make enough thyroid hormone for the cats to be hyperthyroid. In practice, this is hard to achieve and studies show that a high percentage of cats eating only the Y/D diet remain mildly hyperthyroid.Since the diet relies on severe iodine restriction, cats cannot eat anything else that contains iodine or the tumour will produce thyroid hormone resulting in elevated thyroid levels. This includes many things that we may not normally think of as “food” such as treats, bits of table food, milk and even some medications. Cats that go outside may find iodine containing foods and lose control of hyperthyroidism.

Iodine is involved in other metabolic processes and we do not yet know the long term effects of iodine deficiency on the body.

The protein content of Y/D provides only 50-75% of the protein amount currently recommended for senior cats. As hyperthyroidism causes muscle wasting, protein requirements are even higher for hyperthyroid cats making Y/Ds protein levels even more inadequate for hyperthyroid senior cats.

This diet should not be fed to non-hyperthyroid cats as their sole diet which may make feeding difficult in a multi-cat household.

Just like drug therapy, diet therapy allows progression of the disease and transformation into malignant cancers.

As Y/D is protein restricted, it may be useful for cats in late stages of kidney disease who are not good candidates for permanent cure.