Thanks here is a site to help you figure out how many calories her size dog should need
http://www.mycockerspaniel.com/mer.htm
You can do it for the weight she is and also for the weight you wish she was and compare that.
Usualy on the bag it says kcal per cup somewhere. For example Beneful
http://www.beneful.com/dog-food/Original/Default.aspx#shown
says 360kcal per cup of food.
The meats and any fats and carbs add more calories of course.
I'd suggest adding less goodies to her evening meal to help with calorie reduction.
Maybe make it a tablespoon with some extra broth (fat free broth!) mixed into the dry food. The sad thing is some spayed females just need fewer calories than can be believed to maintain weight in the absence of lots of exercise.
You might try a better quality dry food from the pet supply store (not a big box store though) such as Blue Buffalo, Innova, Canidae, Merrick, Eagle etc. again check calories per cup to decide how much to feed.
Or, as you do home cook now, you might check out a dog cook book for balanced home made foods or a recipe like this one
http://tedeboy.tripod.com/drmichaelwfox/id19.html
or these
http://www.heraldnet.com/article/20070331/NEWS01/703310718&SearchID=73286336573052
You can cook and freeze the meals if you want.
If you have not had her blood retested for thyroid level that would be a good choice as it may be she isn't on enough of the med to work for her.
Hope this helps you!
You might find something running a comprehensive blood test to look at other hormones etc, but in general if the thyroid is properly balanced (and it can change as time goes on) the weight is controlled by the amount of calories in vs the amount burned in a day's exercise. You can read about thyroid issues here
http://www.marvistavet.com/html/body_hypothyroidism.html
For many dogs spaying reduces the calories needed as less are burned in the maintenance of the body to be ready to produce a litter.
Do try calculating her calories needed per day against her current weight, and what she should weigh and compare to what she is being fed.
Your situation is different as the pancreas controls digestive enzymes and if the pancreas is not working properly food doesn't digest.
You can read about dogs and weight gain here
http://www.thepetcenter.com/article.aspx?id=3409
You might try cutting back her diet by 1/3 and replacing missing high calorie food with carrots, green beans, or plain canned pumpkin (not pie filling) to give her a full feeling with lower calories.
You might increase her daily walks where you insist she be active by walking with her and playing ball with her etc.
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