Hill's Prescription Diet y/d Dry Cat Food is an easy and convenient way to reduce the iodine levels in food for cats with hyperthyroidism. For best results, consistent long-term feeding of this product is critical for efficacy of the product benefits. Dry and canned formulas can be mixed or fed concurrently. Suitable for: Adult cats. Not suitable for: Growing kittens, pregnant or nursing cats. Recommended for: Hyperthyroidism.