Normal Hair Growth
About 90 percent of the hair on a person's scalp is growing at any one time. The growth phase lasts between two and six years. Ten percent of the hair is in a resting phase that lasts two to three months. At the end of its resting stage, the hair is shed. When a hair is shed, a new hair from the same follicle replaces it and the growing cycle starts again. Scalp hair grows about one-half inch a month. As people age, their rate of hair growth slows. Natural blondes typically have more hair (140,000 hairs) than brunettes (105,000 hairs) or redheads (90,000 hairs). Most hair shedding is due to the normal hair cycle, and losing 50-to-100 hairs per day is no cause for alarm. However, if you are concerned about excessive hair loss or dramatic thinning, consult your dermatologist.

Though humans no longer make use of hair for protection, heat retention, or camouflage, it still remains a very important means by which individuals display and are recognized. Appropriate appearance and grooming are still very important in social organization and the human relationships.

The human body contains approximately five million hair follicles while the scalp (prior to any kind of hair loss) contains 100,000-150,000 hair follicles. Blondes have the greatest number of scalp follicles, followed by brunettes. Humans with red hair have the fewest number of scalp follicles. The normal growth rate of scalp hair is one-fourth to one-half inch per month.

Hair growth is not a continuous process: it has several stages.

ANAGEN PHASE - The first phase is the growing stage. Hair grows at about 1 cm each month, and this phase lasts for anything between 2 and 5 years.

CATAGEN PHASE - As this phase begins the bulb detaches from the blood supply and the hair shaft is pushed up.

THE TELOGEN PHASE - This is followed by a resting stage, during which there is no growth. This phase lasts about 5 months.
At the end of the resting phase, the hair is shed, and the follicle starts to grow a new one.
At any moment, about 90% of the hair follicles of the scalp are growing hairs in the first phase; only about 10% are in the resting phase. If a follicle is destroyed for any reason, no new hair will grow from it.

Dermatologists, physicians who specialize in treating diseases of the hair and skin, will evaluate a patient's hair problem by asking questions about diet, medications including vitamins and health food taken in the last six months, family history of hair loss, recent illness and hair care habits. Hormonal effects may be evaluated in women by asking about menstrual cycles, pregnancies and menopause. After examining the scalp and hair, the dermatologist may check a few hairs under the microscope. Sometimes blood tests or a scalp biopsy may be required for an accurate diagnosis. It's important to find the cause and whether or not the problem will respond to medical treatment.

For as many people as there are that experience hair loss, there are as many reasons for it. No single explanation will suffice for all hair loss, however, here are a few explanations:

Male Pattern Baldness
Most hair loss is caused by your genetic make up, it is an inherited trait. If you posses genes responsible for hair loss when you were born, they made the hair follicles on top of your head sensitive to the hormone dihydrotestosterone, or DHT. These follicles begin to decrease when you first become an adult, as the amount of DHT increases in your body.

This process continues throughout your life and as the hair becomes finer and finer, you will also discover that your hair doesn’t grow as long or as rapidly as it did when you were younger. It’s at about this point that you begin to experience either a receding hairline or a patch of balding scalp on the top of your head.

It’s unusual though that some men with high testosterone levels (marked by heavy beards, an excess of body hair and deepness of voice) do not succumb to male pattern baldness, while others, often with lower testosterone levels, do. Pattern baldness also only affects the hair follicles on top of the head, not those on the back and sides.

While many people believed until recently that you received this genetic message through your mother and that you would eventually lose hair in the same way her father did, it is now accepted that baldness genes are passed down from both sides of the family. This trait can skip generations and it may not even affect siblings in the same way.

Hairstyles that pull on the hair, like ponytails and braids, should not be pulled tightly and should be alternated with looser hairstyles. The constant pull causes some hair loss, especially along the sides of the scalp.

Alopecia Areata - Alopecia areata (al-oh-PEE-shah air-ee-AH-tah) is a highly unpredictable, autoimmune skin disease resulting in the loss of hair on the scalp and elsewhere on the body. This common but very challenging and capricious disease affects approximately 1.7 percent of the population overall, including more than 4 million people in the United States alone. Due to the fact that much of the public is still not familiar with alopecia areata, the disease can have a profound impact on one's life and functional status, both at work and at school.

In this type of hair loss, hair usually falls out, resulting in totally smooth, round patches about the size of a coin or larger. It can, though rare, result in complete loss of scalp and body hair. This disease may affect children or adults of any age.

The cause of alopecia areata is unknown. Apart from the hair loss, affected persons are generally in excellent health. In most cases, the hair re-grows by itself. Dermatologists can treat many people with this condition. Treatments include topical medications, a special kind of light treatment, or in some cases pills.

Childbirth - When a woman is pregnant, more of her hairs will be growing. However, after a woman delivers her baby, many hairs enter the resting phase of the hair cycle. Within two to three months, some women will notice large amounts of hair coming out in their brushes and combs. This can last one to six months, but resolves completely in most cases.

High Fever, Severe Infection, Severe Flu - Illnesses may cause hairs to enter the resting phase. Four weeks to three months after a high fever, severe illness, or infection, a person may be shocked to see a lot of hair falling out. This shedding usually corrects itself.

Thyroid Disease - Both an over-active thyroid and an under-active thyroid can cause hair loss. Your physician can diagnosis thyroid disease with laboratory tests. Hair loss associated with thyroid disease can be reversed with proper treatment.

Inadequate Protein in Diet - Some people who go on crash diets that are low in protein, or have severely abnormal eating habits, may develop protein malnutrition. The body will save protein by shifting growing hairs into the resting phase. Massive hair shedding can occur two to three months later. Hair can then be pulled out by the roots fairly easily. This condition can be reversed and prevented by eating the proper amount of protein and, when dieting, maintaining adequate protein intake.

Medications - Some prescription drugs may cause temporary hair shedding. Examples include some of the medicines used for the following: gout, arthritis, depression, heart problems, high blood pressure, or blood thinner. High doses of vitamin A may also cause hair shedding.

Cancer Treatments - Some cancer treatments will cause hair cells to stop dividing. Hairs become thin and break off as they exit the scalp. This occurs one to three weeks after the treatment. Patients can lose up to 90 percent of their scalp hair. The hair will re-grow after treatment ends. Patients may want to get wigs before treatment.

Birth Control Pills - Women who lose hair while taking birth control pills usually have an inherited tendency for hair thinning. If hair thinning occurs, a woman can consult her gynecologist about switching to another birth control pill. When a woman stops using oral contraceptives, she may notice that her hair begins shedding two or three months later. This may continue for six months when it usually stops. This is similar to hair loss after the birth of a child.

Low Serum Iron - Iron deficiency occasionally produces hair loss. Some people don't have enough iron in their diets or may not fully absorb iron. Women who have heavy menstrual periods may develop iron deficiency. Low iron can be detected by laboratory tests and can be corrected by taking iron pills.

Major Surgery/Chronic Illness - Anyone who has a major operation may notice increased hair shedding within one to three months afterwards. The condition reverses itself within a few months but people who have a severe chronic illness may shed hair indefinitely.

Fungus Infection (Ringworm) of the Scalp - Caused by a fungus infection, ringworm (which has nothing to do with worms) begins with small patches of scaling that can spread and result in broken hair, redness, swelling, and even oozing. This contagious disease is most common in children, and oral medication will cure it.

Hair Pulling (Trichotillomania) - Children and sometimes adults will twist or pull their hair, brows, or lashes until they come out. In children especially, this was often considered just a bad habit, this is not the case at all. Trichotillomania (trich) is said to be a chemical imbalance in the brain. Though not much is known about this disorder or condition, it has been ruled out that it is not a bad habit but a sign of a serious problem needing the help of a health professional.

Medical - Another common cause of hair loss is chemotherapy drugs and radiation treatments. Chemotherapy drugs designed to poison cancer cells also poison the hair follicles and will often result in total baldness. Hair loss from chemotherapy treatments is not permanent and hair will generally grow back within 6-12 months. It may grow back thinner and perhaps a different color, but will eventually return to its original thickness and shade. Certain medications and diseases will also affect hair growth and loss.

Drugs that may cause Hair Thinning:
Iotretinoin Acne Captopril, Enalapril, Lisinopril, Allopurinol
ACE inhibitors (blood pressure or heart failure)
Warfarin Gout
Valproate Sodium, Vigabatrin
Blood-thinning drugs
Carbimazole, Propylthiouracil Epilepsy
Clofibrate, Bezafibrate Thyroid