There are tons of health issues that can affect dogs. Here we are only going to address the ones that shih tzu are prone to (although other breeds and mixed breeds may also be prone to them).

Eye injuries

Due to the shape of their face, shih tzu are prone to injuries of the eye. Most injuries can be prevented by owner vigilance. First you want to clean around the eyes often. Also, by doing this you are able to observe the eyes on a regular basis and can detect an injury early on. The best thing to do is to NOT clip the hair around the eyes. Be sure to inform your vet and groomer not to do this. But if you do this, be sure to keep that hair cut VERY short. When hair starts growing back in around the eyes it can poke into the eyes causing ulceration.

Ear infections

Shih tzu are prone to ear infections. Owners can help to prevent infections by keeping the inside of the ears free of hair and by keeping the ears clean. For info on how to do this, please refer to Grooming Pet Shih Tzu

Thyriod abnormalities

This is where the thyriod gland either over or under produces. Signs are abnormal weight loss or gain, poor coat and infertility. If you suspect that your shih tzu has a thyriod problem, consult your vet. Your vet can run tests to diagnose the problem. If your shih tzu has thyriod problems, it may have to take pills for the rest of its life. Please remember too that thyriod problems are often times genetic so its best if you not breed a shih tzu with thyriod problems.

Hip dysplasia

Despite common belief that only large dogs get hip dysplasia, many shih tzu have been found to have it. Hip dysplasia is where the ball of the hip joint doesnt fit firmly into the socket. Sometimes this can be repaired by surgery. Hip dysplasia can range from so severe the dog cant walk, to so mild that its unnoticable to the naked eye. Hip dysplasia can be diagnosed by x rays taken by your vet. Hip dysplasia is largely genetic, so dont breed any dog found to be dysplastic, no matter how minor the dysplasia is.

Progressive Retinal Atrophy

Progressive Retinal Atrophy, also known as PRA, is a genetic eye disease. PRA can lead to early blindness. If your vet suspects PRA, it is best to get a postive diagnosis from a veterinary opthamologist. Your vet op can give you treatment options. PRA is also genetic, so remove any dog with PRA from breeding.

Renal dysplasia

Renal dysplasia is an inherited form of kidney disease. Symptoms include increased water intake, increased urination and poor health in general. Many vets still think kidney disease only affects older dogs. This is not true of renal dysplasia. Renal dysplasia can affect very young puppies. Mild forms of renal dysplasia may be symptomless. Right now there is an ongoing project to develop a gene marker for renal dysplasia in shih tzu, read about it here: Renal Dysplasia in Shih Tzu

Liver shunt

In a normal liver, the liver detoxifies blood from the gastro-intestinal tract before it goes circulating through the rest of the body. A liver shut bypasses this and the blood goes on with out being detoxified. Liver shunts can be either genetic or a birth defect.

Cleft palates

Cleft palates are something a pet owner normally doesn't have to deal with. A cleft palate typically refers to when the roof of the mouth does not properly close during development. Most puppies born with this either die or are euthanized. Surgery can be performed to correct this in minor cases, but only if the puppy can be kept from dying until it is old enough to undergo surgery. Most puppies will die from this as they will aspirate their mothers milk. Cleft palates can be genetic or may be a birth defect. It is recommended not to breed any dog known to produce this.

Allergies

Allergies is pretty self explanatory. Canine allergies, whether they are inhalant or food allergies, tend to manifest themselves in raw, itchy skin. Constant chewing on the feet can also be a sign of allergies (but it can also be a sign of boredom). It is my personal belief that many allergies are inherited. Also some dogs are allergic to fleas. For dogs allergic to fleas, it can take only one flea to set off a reaction. Your vet can run allergy tests if you suspect your dog is allergic. That test will tell you what your dog is allergic to and based on the test results your vet can help you combat the allergies.

Grooming

I want to conclude by saying that these are things that CAN happen to your shih tzu. Most shih tzu live out their entire lives without being affected by any of these problems. You can decrease your risk of encountering these problems by being a responsible owner and buying from a responsible breeder.