Levothyroxine can have a side effect of causing nausea. If the dog is feeling nauseated then it wouldn't want to eat. You should discuss with your Vet the possibility of trying a lower dose, or if the Vet believes the dose is appropriate, then ask for Metoclopromide. If the problem is nausea then the Metoclopromide should help. Again discuss with the Vet and see what s/he thinks about the appetite problem.