Allergies

Allergies to food, medication, environmental toxins or topical substances (like poison ivy, or sensitivity to certain metals) can result in hair loss. A cat can also be allergic to fleas, and have an exacerbated reaction with hair loss.

Infections

Bacterial infections or infections with flea or mite infestations can lead to hair loss. Folliculitis, or an infection of the hair follicle, also results in thinning of fur. Infections in cuts or scrapes can also lead to balding through excessive scratching and itching. Ringworm infections can cause hair loss as well.

Disease

Cushing's Disease, hyperthyroidism, Malassezia, Alopecia areata and other congenital defects are all conditions that can cause hair thinning or bald spots.

Medical Reactions

Some cats are sensitive to certain drugs, and may have hair loss at injection sites or where topical medications are applied. Hair loss is also seen in cats undergoing chemotherapy.

Other Causes

Glandular blockages, sunburns and neurological issues can cause Alopecia. One example of a neurological issue is excessive licking or chewing on a body part. This often occurs when an animal is anxious or stressed.

Source:

Drs. Foster & Smith: Causes of Hair Loss (Alopecia) in Cats