Because of their small size and lack of a thick undercoat, Yorkshire terriers chill easily. They are not equipped to stay outside in freezing temperatures for long periods of time, nor are they equipped to be wet in an unheated environment. You may find it strange to put clothing on a dog with such a luxuriant coat, but it's necessary to keep your Yorkie warm. Go for practicality over fashion. Look for a sweater or jacket that's easy to put on and take off.
Signs of hypothermia include uncontrollable shivering, lethargy, depression, and finally, unconsciousness. If your Yorkie appears to be suffering from the cold, pick her up and put her inside your jacket to share your body warmth. Once she returns to normal, don't simply put her back on the ground. Swift changes in body temperature can cause stress to your Yorkie's body.
Also keep an eye out for frostbite. Ears, toes, and scrotums are most susceptible, and older dogs with less efficient circulation are at increased risk. Symptoms are the same as for humans — red or pale skin, pain, and itching. Warm the affected area gently, without rubbing.