A consumer writes to ask "What triggers pattern hair loss really?".  Here's my response:

 Your e-mail was forwarded to my attention so I hope you do not mind if I reply personally.  I should preface my comments by noting that I am a Ph.D. research scientist, NOT an M.D. clinician.  This means that any thoughts I offer in this regard should not be construed as medical advice.  As long as we both appreciate the limitations of what I've just conveyed, here are my thoughts.

The most common form of hair loss in women, as well as men, is called pattern hair loss AKA androgenetic alopecia (AGA).  AGA is known as pattern hair loss because hair outside of the pattern is remains essentially unchanged, while hair inside the pattern thins and ultimately weakens and dies.  There are three key factors that influence the disorder. 

The first is age.  You have to reach a certain point in adulthood before the problem occurs.  This is why nine year old children do not typically suffer pattern hair loss. 

The second is genetics.  One must have a genetic predisposition to be affected by AGA.  AGA is also understood be a complex-trait disorder, that is to say a disease which is set in motion by more than one trigger.   Unlike other less-common forms of hair loss which may be monogenic (caused by only one genetic defect), AGA is influenced by multiple genes.  This is why there is such variation in the population, from almost complete loss of hair to basically no loss at all. 

The third component is endocrinologic, i.e. hormonal.  The principle androgen hormone linked to pattern hair loss is dihydrotestosterone (DHT) synthesized from testosterone by the enzyme 5 alpha reductase (5AR).  Think of DHT as the gasoline that fuels the destruction of the genetically susceptible hair follicle and 5AR as the match that sets the gasoline on fire and you have some idea of how this works. 

Thi
s is one reason why most current first line treatments against pattern hair loss seek to block one or more of the steps associated with DHT metabolism.  

Interestingly, because pattern hair loss is known to be a disease which is triggered by by multiple factors, aside from DHT, the potential for designing improved treatment methodologies may well hinge on designing formulations that effectively address the contributory pathological agents, rather than simply trying to figure out various ways to block DHT. 

This has led investigators in our lab and elsewhere to attempt to identify relevant factors in common hair loss, aside from DHT.  As time passes, the newer and better treatments will most likely consist of cocktail mixes where a number of important pathological elements that contribute to hair loss are checked, as opposed to just one (i.e. DHT).