Please let us know what your vet says. In the meantime, I found an article about diagnosing Canine Symmetrical Alopecia (symmetrical pattern baldness in a dog):
Diagnostic approach to canine symmetrical alopecia - Cerundolo - 2013 - Companion Animal - Wiley Online Library
It is behind a pay firewall. Here is the beginning of the article:
Owners and breeders often consult the veterinary surgeon if they recognise that their dogs have even subtle changes in coat density that may or may not lead to alopecia. In some otherwise healthy dogs, the alopecia could be considered a cosmetic disorder, but this may still be a cause of frustration for the owner.
The alopecia could be a manifestation of a cutaneous or systemic disease, with potentially serious consequences, but, if properly managed, the hair will grow back. Initially, the hair loss may be localised; however, if there is an unrecognised underlying cause, the affected areas may enlarge and the alopecia may become symmetrical or generalised.
Symmetrical alopecias are rarely congenital. More commonly, hair loss is caused by one of the following:
• Severe folliculitis (demodicosis, dermatophytosis, superficial pyoderma)
• Abnormalities of the hair follicle anatomy or its cycling (follicular dysplasias, endocrinopathies and alopecia X)
• Self-trauma–often caused by pruritic conditions (allergies) Alternatively, the hair may be so short that it can simulate a classic form of alopecia.
If you are interested in reading more, or recommending this article to your vet, PM me your email address and I will email you a PDF of the article.