A cat's hair can fall out in patches because of hyperthyroidism, ringworm, compulsive grooming, cortisone excess, mange and feline endocrine apopecia, according to WebMD. Feline hair loss is referred to as alopecia and can be partial or complete, according to PetMD. Alopecia also occurs in older cats with cancer and skin allergies. WebMD notes that cat hair loss often makes the coat coarse, brittle or greasy.
Ringworm is a fungal infection that causes cat hair loss, according to WebMD. This condition results in the cat's hair falling out in patches of one-half to two inches across, often with a red ring at the periphery. The skin is often scaly and crusty. Hyperthyroidism — excess thyroid hormone — can cause hair loss in about one-third of cats with the condition. Feline endocrine alopecia is the thinning or balding of the coat on the inside of the back legs. This occurs in neutered males and spayed females. Cortisone excess causes a symmetrical hair loss over both sides of the cat's body, often with a darkening of the skin. Cortisone excess is a likely result of a thyroid problem.
Veterinarians can diagnose the reason for the hair loss, says PetMD. Blood tests, X-rays and muscle biopsies can help determine the cause.