WebMD states that there are several diseases that can cause a dog's hair to fall out in chunks, such as growth hormone-responsive alopecia, hyperestrogenism and hypothyroidism, which are all hormone-related. Non-hormone-related diseases for hair loss in dogs include acanthosis nigricans, which mostly affects dachshunds, as well as color mutant alopecia, demodectic mange, pressure sores and ringworm.
Patchy hair loss is particularly evident in hypothyroidism and hypoestrogenism, according to WebMD. The former is accompanied by a thin coat. The latter is accompanied by scanty hair growth and smooth skin and occurs only in females that have been fixed. Hair loss due to growth hormone-responsive alopecia occurs symmetrically and then only at certain times in the dog's life and in certain breeds.
WebMD says that parasites are the most common cause of patchy hair loss. For instance, ringworm can cause hair to fall out in chunks while also producing scaly, red circular patches on the skin. Demodectic mange, an immune deficiency disease, results in patches of hair loss, especially on the face. Color mutant alopecia is a disease that not only results in patchy hair loss but is also accompanied by pustules. Pressure sores occur in large dogs and result in hair loss because the animals lie down on hard surfaces, negatively impacting pressure points.