Dogs constantly lose hair, but extreme thinning of the hair, or even baldness, isn't considered normal. A trip to the veterinarian may be required to determine what the underlying cause of the hair loss is and to identify potentially serious health conditions.

Hormone problems

Adrenal gland diseases can cause hormone imbalances in your dog, which may lead to hair loss. Cushing's disease causes too much cortisol to be produced, resulting in hair loss, as well as other symptoms, including swollen abdomen, increased hunger and thirst, as well as muscle wasting. Another adrenal disease causing hair loss is alopecia X, which causes your dog to produce excessive amounts of sex hormones.

Allergies

Your dog can suffer skin irritation from a broad range of environmental and/or food allergens. The resulting dermatitis can manifest with hair loss. Inflammation, and subsequent itching and scratching, can cause your dog's hair to break off or fall out.

Parasites

If your dog is suffering from hair loss, along with scaly, blistered, itchy skin, he may have a form of mange. There are three types of mange -- demodectic, sarcoptic, and Cheyletiella. Dogs with mange must be seen and treated by a veterinarian.

Parasites

A fungal infection, such as ringworm, can cause pattern-like hair loss on your dog. Other symptoms include scaly, itchy, crusty skin; again, a condition that requires your dog to be treated by a veterinarian.

Psychological

Stress or boredom can create physical symptoms in your dog. Dogs that are stressed may develop patches of bare, oozing skin that are inflamed and may itch or hurt. If your dog is bored, or anxious, he may develop acral lick dermatitis (obsessively licking himself), which results in hair loss.

Source:

Vetinfo.com: Five Common Causes of Dog Hair Loss

2ndchanceinfo.com: Why is My Dog Losing Hair?

The Complete Healthy Dog Handbook, March 2009