Underlying Health Conditions Being underweight can also signal an underlying illness, especially if the weight loss is sudden. A number of health issues can cause weight loss in dogs, ranging from tooth pain to gastrointestinal discomfort to more serious conditions such as diabetes, thyroid disease and cancer.
If you suspect your dog is too skinny, here are some signs you can look for, according to Rover:
The ribs are clearly visible or feel like they lie just beneath the skin, with no fat layer in between.
The hip bones, shoulder bones and spinal cord are easy to feel beneath the skin.
The bones at the base of the tail are protruding.
The curve between the ribs and hips looks extreme when viewed from above.
The spine and ribs are plainly visible when viewed from above.