Hair follicles on the scalp do not continuously produce hair. They cycle through a growth stage that can last two or more years, then regress to a resting stage for up to two months before starting to grow a new hair fiber again. At any time on a healthy human scalp, about 80% to 90% of the hair follicles are growing hair. These active follicles are in what is called the anagen phase. That leaves up to 10% to 20% percent of scalp hair follicles in a resting state called telogen, when they don't produce any hair fiber.
Telogen effluvium (TE) is probably the second most common form of hair loss dermatologists see. It is a poorly defined condition; very little research has been done to understand TE. In essence though, TE happens when there is a change in the number of hair follicles growing hair. If the number of hair follicles producing hair drops significantly for any reason during the resting, or telogen phase, there will be a significant increase in dormant, telogen stage hair follicles. The result is shedding, or TE hair loss.
TE appears as a diffuse thinning of hair on the scalp, which may not be even all over. It can be a bit more severe in some areas of the scalp than others. Most often, the hair on top of the scalp thins more than it does at the sides and back of the scalp. There is usually no hair line recession, except in a few rare chronic cases.
The shed hairs are typically telogen hairs, which can be recognized by a small bulb of keratin on the root end. Whether the keratinized lump is pigmented or unpigmented makes no difference; the hair fibers are still typical telogen hairs.
People with TE never completely lose all their scalp hair, but the hair can be noticeably thin in severe cases. While TE is often limited to the scalp, in more serious cases TE can affect other areas, like the eyebrows or pubic region.