The more I research, the more I learn and understand: This is from Royal Canin Weight management. Good flowchart and descriptions.
http://www.pet-slimmers.com/shapedog.htm
and another from OSU (Ohio State University)
http://vet.osu.edu/1851.htm better visuals and diet search tool
http://vet.osu.edu/1442.htm
My main concern is that underweight Yorkies, esp young ones, may have problems with hypoglycemia, or I believe that hunger can influence behavior also. It seems like many of you are feeding a kibble diet only. If your dog/pup has any kind of tooth problems (pain) this will prevent them from eating enough and may be just what could be preventing him/her from gaining a little bit more weight.
Here are somes tips:
Offer moistened kibble or canned food twice a day. The added water will help prevent dehydration also, and provides a bit of variety. If they eat alot of the softer food, maybe the vet should have a look in their mouth to rule out any problems.
Try different sized kibble. Some like bigger, some smaller. Some don't like kibble at all. Pop some in a blender to pulverize it and add it to their soft food diet, gradually make it bigger until they accept it.
Measure how much you feed, but look at your dog. If he/she is thin, try to get them to eat a little bit more, even if it's a treat or two for eating their food. I even hide treats at the bottom of the bowl or mix it in. One vet on a website said: Eating some of the wrong diet is better than eating none of the right diet. Just something to keep in mind.
Do not feed them people food from your table. These dogs are very smart and even after only one transgression, they will not eat their own food until your food is gone, thinking they will get yours.
How much they need to eat depends on their lifestage and activity level. This changes many times throughout their lifetime. So their weight should be constantly monitored. Most of the guidelines on pet food packaging states to feed according to weight and adjust as necessary. If your dog is gaining too much, reduce the amount fed. If your dog begins to lose weight or looks thin, increase the amount fed.
Sometimes a walk or exercise will stimulate their appetite. Sometimes a t-shirt can help a hyper dog to calm down (and burn less calories). I do this with my LS dog. So far it works!
Internal parasites can cause weigh changes in pets. Illness can cause changes, like diabetes, Liver Shunt, MVD, Kidney/renal problems, dental disease, and their weight can be an indicator to get them to the vet.
I also think that metabolism plays an important role. Using Probiotics such as
http://www.naturesfarmacy.com/store/details.php?prodId=198&category=30 can boost metabolism, by helping to more completely digest the food allowing better absorbtion of nutrients in the body.