**5. Functional types of telogen effluvium**

Headington defined five types of functional TE, according to the different follicular cycles. These include the immediate anagen release, delayed anagen release, immediate telogen release, delayed telogen release, and short anagen phase [2].

#### **5.1. Immediate anagen release**

It is a common form of TE which is related to physiological stress, severe illness, and drug use. During stress, the cytokines induce apoptosis of hair follicle keratinocytes, first with catagen, followed by telogen [2]. Therefore, follicles, which are induced to leave the anagen, enter telogen early [14].

#### **5.2. Delayed anagen release**

It typically occurs in women with postpartum hair loss, and when the oral contraceptives are discontinued. It is also known as telogen gravidarum. It is caused by high levels of circulating placental estrogen, which prolongs the anagen phase, result in a full head of hair during pregnancy. The withdrawal of these hormones during delivery stimulates the overdue anagen hair to enter into the catagen phase simultaneously. As a result, an increased shedding of telogen hair can be seen after a couple of months of delivery [2, 15].

#### **5.3. Immediate telogen release**

Drug-induced shortening of telogen results in follicles with the reentry of the anagen prematurely [14]. Hair follicles typically release the club hair 100 days later. It is caused by a shortened normal telogen cycle. This type of hair shedding usually occurs 2–8 weeks, following therapy with topical minoxidil [16]. As the exogen hair at resting is released, this paradoxical phenomenon occurs, by stimulated anagen phase [2].

#### **5.4. Delayed telogen release**

Hair follicles do not shed or recycle into anagen, but remain in prolonged telogen. When teloptosis defined as the termination of telogen phase with hair shedding occurs, the main clinical manifestion of increased shedding of the club hair presents. In such cases, the major cause is seasonal hair loss [2, 16].

#### **5.5. Short anagen phase**

Idiopathic shortening of anagen duration results in persistent telogen shedding. The condition is not associated with the hair shaft fragility or hair unruliness. It leads to resistant and chronic TE. It is common in hereditary hypotrichosis and ectodermal dysplasia and as an isolated disorder in otherwise healthy children [2, 16].
