With my old girl, she was actually sensitive to anything beef. Beef, beef by products, animal fat, animal by products...as a result she was skinny, even though she ate plenty, and had terrible skin/hair. I managed to get her switched to a food with nothing that she had problems with and she picked up her weight and her coat was beautiful! I 100% believe that most of the problems are CAUSED by sh**** foods filled with grains that are fed from such a young age. Even my kitten, that I got from his breeder, was being fed IAMS (yuck). He's a Ragdoll and is supposed to have a gorgeous, silky coat but it was rough and yucky. Switched him to a grain free diet and everyone always complements his silky coat. Same with the dog. He looks like his coat should be kind rough but it is silky and soft. I am sure it's the quality of food he gets! I temporarily had a cat from the shelter, probably a Maine Coon. Her coat was terrible, got her on a quality food and she got silky and shiny. No need for supplemental oils.
Now, all that said. A blood panel is not
a bad idea at all. Especially at that age. Food is probably a huge factor, but that may not be all of it!!