Category Archives: Hair Growth Cycle

Kenogen: A New Phase of the Hair Cycle?

Earlier this month, an interesting new study was published in the Experimental Dermatology journal by UK based scientists. The title of this study was “Hair regrowth in male and female pattern hair loss does not involve the conversion of vellus hair to terminal hair“.

Basically, the scientists found that when medications such as minoxidil, finasteride and anti-androgens were used to treat hair loss patients (both men and women), the resulting hair regrowth was almost entirely attributable to the reactivation of dormant non-vellus hair follicles (termed as “kenogen” hair follicles) rather than the conversion of fine miniaturized vellus hair into thick terminal hair.

This is a very surprising finding since in both male pattern hair loss (MPHL) and female pattern hair loss (FPHL), terminal hair miniaturizes via shorter and shorter growth cycles and finally becomes vellus almost invisible fine hair. I am pretty certain that it has long been assumed that hair loss medications cause some of these vellus hair to become terminal hair once again (if I have time later this week, I will try to find studies in support of such a theory). However, this latest study suggests something entirely different by concluding that:

We would propose there is a population of growth restricted (dormant/kenogen) non-vellus hair follicles, which are re-activated by effective medical treatments as an explanation for the increased hair growth observed in FPHL and MPHL. Our findings have a fundamental impact on the pathophysiology of hair changes occurring in patterned hair loss.

Hair Follicle Growth Cycle and Phases

We have all heard of the three main phases of the hair follicle growth cycle in anagen (growth), catagen (transition/regression) and telogen (resting). There is also a lesser known phase called exogen, which is when hair follicles undergo shedding. Also see my post on telogen effluvium.

However, I had never heard of this new kenogen phase before. A basic google search showed that while this phase is almost never mentioned in literature, it was discovered as early as in 2002 when a study titled “Kenogen. A new phase of the hair cycle?” was published in Italy.

I am not sure if just several studies on this are sufficient to make any conclusions, but it is hard to believe that there could be some follicles in a dormant state that were previously unaccounted for and that are being reactivated by hair loss medications.

Telogen Effluvium Related Hair Loss

What is Telogen Effluvium?

Telogen Effluvium in a Woman
Telogen Effluvium in a Female.

Telogen effluvium (TE) is a condition that involves temporary loss of large quantities of hair. It is due to shedding of telogen (resting phase) hair. This loss of hair typically arises as a result of some major shock to a person’s biological system.

An effective treatment for telogen effluvium will first and foremost need to tackle the initial cause of the shedding. Medications and natural hair loss treatments for telogen effluvium will not work unless the underlying cause is not addressed first.

Among events that can cause telogen effluvium include:

  • Surgery.
  • Illness, especially when including fever.
  • Sudden extreme weight loss.
  • Stress and panic attacks.
  • Psychological disturbances.
  • Drastic changes in dietary and eating habits, including crash diets.
  • Traumatic life events.
  • Hair cycle disorders.
  • Lupus.
  • Hormonal fluctuations such as those accompanying menopause.
  • Thyroid disorders (both hyperthyroidism and hypothyroidism).
  • Vitamin deficiencies (e.g., vitamin D and vitamin B12).
  • Side effects from taking new medications or stopping old ones.

Note that the average person naturally loses around 100 scalp hairs every day. In TE, the rate of hair loss is at least several times larger than what would be normally expected. The hair pull test will usually be positive in patient’s suffering from TE. Diagnosis methods can also include the use of a trichogram, trichoscopy and biopsy.


The first signs of telogen effluvium are often when you notice shedding of a lot more hair than usual. Your pillowcases, bedsheets, shower drain, floors and hairbrushes will all have more hair on them that normal. Note that the period of dramatic hair loss associated with TE typically occurs approximately 2-3 months after the triggering event. The pattern of hair loss associated with TE is diffuse and nonscarring. Typically, once you lose 50 percent of your hair, telogen effluvium will not progress any further.


The vast majority of people completely recover from telogen effluvium hair loss. However, it typically takes around six months to see full recovery and regrowth. Moreover, one has to identify the root cause of the initial shedding and make sure that this is addressed immediately.

Phases of the Hair Growth Cycle

Hair Growth Cycle
Hair Growth Cycle Phases.

In most humans, around 85 percent of scalp hair is actively growing, and therefore said to be in anagen phase. A typical scalp hair is in anagen phase for 2-7 years, with the exact duration dependent upon many factors, including: age, sex, ethnicity and overall genetics. In contrast to scalp hair, eyebrow hair has a much shorter anagen phase of just around one month.

Once hair has completed its growth phase, it enters a brief transitional stage called catagen. This phase lasts for around 2-3 weeks, during which the hair converts into a club hair. This conversion process occurs when the blood supply to the hair follicle is cut off.

Finally, once the hair become bulbous with a club-shaped tip, it enters what is known as the telogen or resting phase. During the telogen phase, the dermal papilla no longer supplies any nutrients to the the hair follicle. The telogen phase lasts for around 3 months. Thereafter, hair re-enters into anagen phase and the whole cycle repeats.

Telogen Effluvium versus Anagen Effluvium

Note that the hair loss condition that is known as anagen effluvium is not the same as telogen effluvium. Anagen effluvium is a condition in which hair starts shedding during the anagen (growth) stage of the hair cycle due to interruption of the hair growth process. In contrast, telogen effluvium arises during the telogen (resting) stage of the hair cycle.

Anagen effluvium also leads to far greater and rapid hair loss in comparison to telogen effluvium. The main reason for this is due to the fact that there are far more hairs in anagen phase than telogen phase as outlined earlier. Anagen hairs have a pointed or tapered tip and feathered roots.

Note that telogen effluvium falls under ICD-10 L65.0 under the 2016 International Statistical Classification of Diseases and Related Health Problems.