It happens to millions of women and men.  It's so frequent, in fact, that there's a billion dollar industry of "solutions" to the problem.  We're talking hair loss, and it's no laughing matter.

Normally, our hair grows in a cycle.  When hairs are in their growth phase, they are called anagen hairs.  When they are resting (not growing), they are called telogen hairs.  Catagen hairs are transitioning from growth to rest.  Anagen hairs grow for about 3 years on average (the range can be between 2-6 years).  The transition phase lasts about 1-2 weeks, and then hairs rest for 3-5 months.  They shed, and the cycle starts all over again.  At any given time, if you pluck a number of hairs from the scalp, about 85% will be in the growth phase and 10-15% will be resting.  Less than 1% will be in the transition phase.  We can tell this by the shape of the hair and follicle when it's examined under the microscope.

It's normal to shed 100-150 hairs daily.  Think about it- that's a lot of hair.  And even more so on days that you shampoo.  But sometimes, that number increases, and the hair starts thinning or revealing bald patches.  So let's look at some reasons why people lose their hair.

Pregnancy.  During pregnancy, the anagen (growth) phase is often prolonged and many women enjoy thicker, fuller hair.  On delivery of the baby, however, many follicles change into the telogen phase, and then are shed simultaneously, resulting in thinning/loss of hair 3-5 months after baby comes.  This process is known as telogen effluvium.  And it's one more reason to give your mom a hug-- she brought you into this world and then endured hair loss!

Iron Deficiency.  Low iron levels (seen in a blood test), sometimes caused by mensturation or gastrointestinal blood loss, can also cause a telogen effluvium.  Nutritional deficiencies are often treated with vitamin supplements and a varied, healthy diet. 

Thyroid Deficiency.  Variations in thyroid levels can often cause telogen effluvium as well.  Levels can be checked with a simple blood test.  Thyroid medications, usually monitored by an endocrinologist (doctor who specializes in hormones) can help. 

Medications.  Lots of medications can cause hair loss.  Drug-induced telogen effluvium has been reported with amphetamines, blood pressure medications (captopril, metoprolol), antipsychotics (lithium), retinoids (etretinate), and more.  Changing/discontinuing medications can help.

Androgenetic alopecia is another term for male-pattern and female-pattern baldness.  In this condition, a hormone called dihydrotestosterone affects the androgen receptor gene to cause hair loss.  There is likely a genetic component.  Men tend to show a receeding hairline with loss of hair on the top of the scalp.  Women tend to get thinning along the middle part of the hair in a "Christmas tree pattern."  Several treatments are available, such as rogaine, and for men, finasteride (propecia) is FDA-approved. 

Other conditions that can lead to hair loss include fungal infections, autoimmune diseases (such as lupus or alopecia areata), psoriasis, bacterial infections, and syphilis.  Trauma to the hair from tight ponytails, frequent relaxer and hair color use, and overstyling can also lead to hair loss.

Often blood tests and scalp biopsies are useful to find the cause of the hair loss.  Dermatologists are expert in skin, nails, and hair, and can help!