**2. Definition**

Telogen effluvium (TE) was first termed by Kligman to define an increased shedding of normal club hairs based on the hypothesis that, irrespective of the cause, the follicle tends to act in a similar manner undergoing a premature termination of anagen and precipitating telogen [3].

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Telogen effluvium is a noninflammatory disease characterized by diffuse loss of telogen hair, caused by any disruption of hair cycle which leads to increased and synchronized telogen shedding [4]. It is the leading cause of diffuse hair loss. However, the actual incidence of the disease remains unknown [2]. It has been suggested to result from an abrupt shift of large numbers of anagen hairs to telogen hairs on the scalp with altered ratio of anagen hair to telogen hair from the normal ratio of 90:10–70:30 [1]. The degree of telogen effluvium depends on the severity and duration of exposure, but not the type of the agent [5].

Although the effect of age also remains unclear; elderly women are reported to be more vulnerable to acute TE following high fever, surgical trauma, severe hemorrhage, or immense psychological stress. In children, TE is responsible for only a minority of cases with hair loss [6, 7].

Telogen effluvium can be classified into two groups, according to the duration of disease, as acute and chronic. The duration of disease is shorter than 6 months in acute TE, while it takes more than 6 months in chronic state. In acute TE, hair loss typically begins 2–3 months later than the triggering event, although the triggering factor is unknown in 33% of the cases. However, chronic TE is frequent in healthy females in the fourth to fifth decades [7]. In addition, telogen gravidarum is a variant of TE which follows childbirth. Significant telogen gravidarum can affect a third to a half of all women following childbirth [8]. In postpartum TE, follicles remain in a prolonged anagen phase, rather than cycling into the telogen phase. Once released from anagen, an increased shedding of telogen hair is one of the clinical signs of the disease [9].
