Hi, this is Dr. Garrett. With the information you have given me, I would say nothing is too serious, but I am glad you will take him to your veterinarian soon. Shih-tzus are prone to a number of ageing issues including back problems, & that certainly can account for intermittant tremors. Hopefully a hands on exam with your vet will provide answers, so relax until then. If there are any further symptoms before your appointment, please let me know. Thank you.
Do you consider his behavior "nothing too serious" because he is not exhibiting any other symptoms other than the trembling?
Yes. We see a lot of small dogs that tremble occassionally and we never can find an exact cause. But sometimes, especially with the long backed dogs, such as your breed, we will find evidence of some back pain that is not apparent everyday, is not bad enough to cause extreme pain, but just enough to cause discomfort. Shivering, without fever or other signs of illness can be your dogs subtle way of saying something is not right. So back pain or another undiagnosed problem can be serious, but at this point in time I still feel you are fine to wait until you see your vet.