There are many reasons for hairless armpits, such as genetic factors, age factors, congenital factors, diseases and so on. 1. Genetic factors: If parents have hairless armpits, the chances of their children having hairless armpits will also increase. 2. Age factor: Hairless armpits may be due to the fact that the body is not fully developed at a relatively young age. Some people will have hairy armpits later, and they may wait until they are 15 years old before they will have hairy armpits, so they have hairless armpits when they are young. 3. Congenital factors: women with congenital ovarian underdevelopment and men with congenital testicular underdevelopment can lead to the phenomenon of hairless armpits. 4. Disease: If you have hairy armpits but suddenly have hairless armpits, it may be due to baldness, sex hormone secretion disorder, hypothyroidism, testicular feminization syndrome, endocrine disorders and other factors. There are many reasons for hairless armpits, so it is recommended that patients go to the hospital for a complete examination and take standardized treatment after a clear diagnosis.