A lack of visible hair growth occurs when your hair follicles continue to follow the normal hair growth cycle, but the growth is then offset by some other factor. Hair loss occurs when the hair growth cycle itself is disrupted, and comes in two forms.
In temporary hair loss, something pushes all or some hair out of the anagen, or growth, phase, and into the telogen, or shedding, phase. Hairs are cut off from nutrients, “killing” the hair, which then subsequently falls out. Once whatever caused hair follicles to shift to the telogen phase is removed, the hair growth cycle resets and normal growth recommences with no lasting damage. These types of hair loss are usually known medically as a type of “effluvium” (or outflow), such as telogen effluvium or anagen effluviem.
In permanent hair loss, the hair follicle itself is damaged, and either slowly or quickly loses the ability to grow new hair, period. Temporary hair loss can become permanent if the catalyst is not removed, and whatever interrupted the follicle slowly destroys it. Or, some problem can directly and permanently destroy the hair follicle. These types of hair loss are classed as “alopecia.”