a good natural shampoo for allergies is dr. bronners unscented mild baby shampoo and it is for pets also. It is all organic oils and castille soap and very safe. I use it on my allergy yorkie too -- there is a group for allergies that is very helpful called
allerdog@yahoogroups.com. I is very helpful with this as i have been dealing with this for 4 years and into it about 5k with dermatologists and specialists. She is hypothyroid, mvd and severe atopic dermatitits and allergies start to occur at ages 6 mos to 3 years in yorkies and usually not food and in most cases environmental which is more difficult to treat. You are lucky if it is just food related. The best allergy diet with minimal ingredients is natural balance fish and sweet potato as it is lower in protein than duck which is important for yorkie breed.