My small dog is losing weight. Is this a serious health issue?

Hello, and thanks for using Petco Pet Education Center, formerly Petcoach. Yes, this unexplained weight loss in Mia definitely worries me. 2 lbs of loss in a large dog wouldn't be too alarming, but in Mia's case she's lost 25% of her original body weight and that should absolutely be investigated. It is never normal for a dog to lose this much weight and almost certainly indicates a medical problem. I recommend bringing Mia to the vet for evaluation. Anticipate that your vet will likely start with an exam, then will probably recommend checking a fecal sample and running bloodwork. Dramatic weight loss in young dogs can be due to problems with digestion (such as inflammatory bowel disease or pancreatic insufficiency), congenital issues (such as a liver shunt), intestinal parasites, and (less commonly) cancer, among other things. I hope this helps and you can get to the bottom of Mia's weight loss soon!