If you see more cat hair around than usual and you’ve been vacuuming and dusting as frequently as always — or if you’re seeing bald spots on your cat — take him to the veterinarian. A cat can lose more hair than normal because of medical or dietary issues.
Precipitating medical issues can include allergies, ringworm, bacterial infections, fleas, hormonal imbalances such as hyperthyroidism, and even sunburn. An inadequate diet — too few calories or an imbalance of nutrients — could cause excess shedding as well. The doctor may want to review your feeding choices for your pet.
Stress can cause a cat’s hair to fall out, too. If someone has moved into or out of your home or if other environmental stressors have gotten the better of your pet, his coat may thin.
Note that many indoor cats shed all year long. There’s no particular seasonality in it. But an indoor cat will not generally shed more hair during one season than another, which is why more shedding than typical at any time of year is a cause for concern.