There are a few different reasons why cats can lose hair. In older cats diagnosed with cancer , alopecia is common. Nervous disorders (., over-grooming) can also cause cats to lose their hair. Hormonal imbalances, specifically too much thyroid or increased levels of steroids in the body, may lead to hair loss. Some cats experience skin allergies, which can also cause hair loss to occur. Parasites that bring about mange , and fungal issues like ringworm, are also a common cause of alopecia. Another less common factor is heredity .