Scalp hair loss is a universal obsession of both sexes.  Its causes are shrouded in mystery and its treatments are based more on magic than on science.  If afflicts men more than women and increases with age, sparing almost no one if one lives long enough.  Because heavy scalp hair is a sign of youthful beauty, enormous amounts of money are spent on styling it, rejuvenating it, and preserving it.  This article will explore in some detail what is currently known about hair growth, hair loss, and hair treatments.
         Scalp hairs cycle through four sequential phases and these natural cycles recur 10 to 30 times during the life of each individual hair follicle.  The hair follicle is like an onion bulb that grows underground but sends its offshoots above ground.  The health of the offshoot mirrors that of the bulb.  When the bulb is healthy the offshoot is robust; when it is sick the offshoot is sick; and when it dies the offshoot disappears.
         a) The first cycle is the Active Growth Phase or Anagen and it lasts between 2 to 8 years each time.  During this phase the hair follicle is highly active and produces a healthy, robust hair.
         b) The second cycle is the Involution Phase or Catagen and it lasts between 4 to 6 weeks.  During this phase, the hair follicle begins to shrink, preparing itself for a well-deserved rest after so many years of sustained productivity.
         c) The third cycle is the Resting Phase or Telogen and it lasts 2 to 3 months.  During this phase, the hair follicle goes dormant much like a dry onion sitting underground, waiting to sprout in spring.
         d) The fourth cycle is the Regeneration & Shedding Phase or Exogen where the young sprouts of hair push out the dead, old Resting Phase hairs and replace them with healthy new ones, thus heralding another prolonged Active Growth Phase.
         Each individual hair follicle cycles independently of all others as if it possesses its own biological clock.  Consequently, while certain hair follicles might be productive, others might be involuting, resting, or shedding.  Therefore, because our hair follicles are not synchronized, the density of hair on our scalps depends on the balance among the growing, involuting, resting, and shedding follicles.  Any disease or disorder that undermines this intricate balance can result in a disproportionate amount of hair loss.
         Normal scalps have about 100,000 hair follicles each, out of which 10% to 15% are usually in the Resting Phase.  We normally shed about 100 to 150 hairs per day as follicles in the Resting Phase awaken, begin to regenerate, and start to push out (shed) the old dead hairs.
         The commonest type of hair shedding occurs when certain diseases or disorders cause the productive hair follicles to cycle out of the Active Phase and go into a Premature Resting Phase or Telogen Effluvium.  When this happens, and since the Resting Phase naturally lasts 2 to 3 months, hair shedding does not begin till after the 2-3 months had passed.         
         Triggers of this kind of hair shedding are nonspecific and include stress (physical or emotional), infections, and the many medical disorders that affect the thyroid, liver, kidneys, blood, skin, gastrointestinal tract, and immune system.  Other causes include deficiencies (of vitamin D, zinc, & iron), sudden weight loss (due to malnutrition, malabsorption, cancer, or chronic illnesses), and medications (such as hormones, blood pressure pills, blood thinners, antidepressants, contraceptive pills, anticonvulsants, and all kinds of nutritional supplements).
         If the causes of the Premature Resting Phase can be reversed, the hair shedding is followed by hair growth.  When no cause can be found— which is common—or when the causes cannot be reversed, permanent hair loss ensues because the Resting Phase is not followed by the usual Regeneration Phase, which leads instead to the death of the hair follicles.
          Male Hormone Hair Loss, which occurs in both sexes, is due to the action of the male hormones (generated by the testicles or ovaries) on the male and female hair follicles—an action regulated by aging and genetic inheritance.  Early treatment with male hormone blockers such as Finasteride or with hair follicle stimulants such as Minoxidil may help but treatment after age 50 is usually of little benefit.
         Alopecia Areata (the autoimmune scalp disorder that causes patchy hair loss) and the many other complex hair loss disorders may require a scalp biopsy for proper diagnosis and treatment.  Age related hair loss due to the irreversibly prolonged Resting Phase respond poorly to treatments and may require hair transplants as a last resort.  Sudden hair loss after cancer treatments is usually reversible in time.  Shaving the head is not a cause of hair loss nor does it strengthen hair growth.  Consulting a dermatologist on complex hair disorders is prudent because early intervention may prevent the hair loss from becoming permanent.

This information is not intended to replace the personal physician, who should always be consulted before any treatment or action are taken.