M-shaped hair loss is typically seen in androgenetic alopecia, also known as male pattern baldness, which is the most common type of male pattern baldness, but can also occur in women. It is highly hereditary and is characterized by progressive hair loss, primarily affecting the hair in the center of the head. The onset of the disease usually begins at puberty, when the hair begins to thin from the sides of the forehead, and it becomes thin and thin, gradually extending towards the top of the head, with the frontal hairline receding backward and the hair on the top of the head gradually beginning to fall out. As the disease progresses, the forehead becomes taller and the hair becomes M-shaped.