Most thyroid medicine for dogs will begin to work immediately. However, it may take several weeks or even a few months for your dog’s symptoms to disappear. For example, it may be a few weeks before you notice your dog beginning to lose weight and their fur filling back in after they start a hypothyroid medicine. Your vet will typically want to perform a blood test a month or so after starting medication to ensure your canine’s thyroid levels are at a healthy level.