As your dog ages, the area of hairlessness may enlarge naturally since he will be growing. This is not to be confused with a progressive skin disease where you would see progression of hairlessness. If your dog is full grown, the area of no hair should not get any larger, if it does, then you are dealing with a different condition.
There are a few conditions that can present with similar symptoms that your veterinarian will need to rule out. Other health conditions such as hypothyroidism, hyperglucocorticoidism, and even sex hormone endocrinopathies can all cause bilateral symmetrical alopecia like you may see in ectodermal defects. Collecting a verbal and medical history of your dog will assist your veterinarian with her diagnosis. When the condition originated will be important when coming to a conclusive diagnosis. If a history is not available, then lab work will need to be conducted to rule out other causes.
Lab work may include a thyroid test to check if his symptoms are secondary to hypothyroidism. Other blood work such as a chemistry panel and complete blood count (CBC) can give general information about your pet’s organ function and general overall health.
Another diagnostic test your veterinarian may recommend performing is a skin biopsy. She would take a skin sample from an area of complete alopecia and one with normal hair growth. She will check for changes in the hair follicle size and density between the different samples. In cases of ectodermal defect, in areas with no hair, the density of hair follicles is decreased and/or smaller. There have even been cases reported where there are no hair follicles in the area at all.