My gsd has been to the vet, blood work completed he is perfectly healthy he is just under weight, he eats a very hi protein diet about 2500-3000 caleries a day and still don't gain weight.. he weighs about 60lbs. He is very active. As long as he doesn't lose weight. I chalk it up to beiing active

GSDs have a laundry list of genetic disease, one being Exocrine Pancreatic Insufficiency

German Shepherd Exocrine Pancreatic Insufficiency | EPI | Chronic Pancreatic Insufficiency | Maldigestion | Causes, Symptoms, and Treatment

Maybe this will help you?

I've had GSD my entire life - some have been heavier than others. The dog I had before this one was very thin his whole life. He ate well, was healthy, lived to be 12, but he remained on the very slender side. I worried - and then one day I saw his littermate, and she was exactly the same! I think sometimes it's genetic.

And, Lucky, thanks for the link - I have never heard that before, despite a lifetime of owning GSD.