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A 48-year-old woman writes that she has been "experiencing hair loss" for the past two to three years and feels challenged because she knows that she is not ill but "many days, when looking in the mirror, [I] feel that I am sick." She's looking for answers as to what causes hair loss and what she can do about it.

Hair is such an important identifying trait for us. Long or short, curly or straight, for many of us, it becomes part of who we are. When I lost mine while having chemotherapy, it had a significant impact on my self-esteem.

To answer our reader's question, I spoke with Paul Cotterill, a hair-restoration surgeon who works in Toronto and New York. As he points out, hair loss is caused by many factors, including, but not limited to, genetics.

"Many people incorrectly believe that if their father is bald, then they likely will be bald as well," he said. The truth is that while much of hair loss is based in genetics, "having a bald father or bald maternal grandfather does not necessarily mean that you will be bald."

But it's true that if many of your family members are affected, particularly on your mother's side, it is more likely that you will lose your hair, too.

At any given time, we have about 100,000 hairs on our scalp. Our hair has an asynchronous growth cycle, meaning that individual hairs are in different phases of growth. At any given time, most of the scalp hair is in the anagen (growth) phase, which lasts about two to four years and during which hair grows at about a half inch a month.

Meanwhile, about 10 per cent of the hair is in the telogen (or dormant) phase, which lasts about three months. During that phase, about 150 to 200 hairs are usually shed each day, while another 150 to 200 hairs go back into the anagen phase. So we are constantly shedding some hairs, while others are in the growth phase.

As we age, men and women alike, we lose more hair. By the age of 70, at least 80 per cent of Caucasian men will show some signs of male pattern hair loss (MPHL), in which hair is lost from the central portion of the scalp, leaving a fringe around the sides.

Hair loss in men with MPHL may happen any time after puberty but usually starts by age 40. As many as 14 per cent of healthy boys between the ages of 15 and 17 may show early signs of MPHL. Usually men who develop MPHL have a history of it on either side of the family; but about 20 per cent of men who develop it do not have a family history of it.

MPHL is both genetically determined and dependent on androgen (testosterone). While it is not clear what the exact genetic mechanism of the hair loss is, it is known that in men who are so genetically disposed, testosterone, even at normal levels, targets the hair follicle and leads to its loss.

Women are also subject to hair loss, known as female pattern hair loss (FPHL), which usually appears as overall thinning throughout the central area, but not the marked centre-spot baldness seen in men.

FPHL can begin any time after the onset of the menstrual cycle, but often doesn't begin until a woman enters her 30s or 40s. Women with FPHL are less likely than men to have a parent or sibling who has hair loss.

Women's bodies also produce small amounts of androgen, but the role of androgen in female hair loss is less certain than testosterone is in men. Some women with FPHL have high levels of androgen, but the vast majority of women with hair loss have normal levels.

Whether you are male or female, if you're experiencing heavy hair loss, you should see your doctor for a complete evaluation. Some medications, such as anabolic steroids or supplemental androgen, might lead to hair loss. So can iron deficiency, or an underactive thyroid gland. A doctor will also want to check a woman for possible androgen excess (other symptoms of which include hirsutism, acne, irregular periods or abnormal breast discharge), or hypersensitivity to testosterone.

Treating hair loss, in both women and men, includes avoiding hair-care products that are damaging to the scalp or hair itself. It's also important to follow a good diet, with adequate protein, which is essential for healthy hair growth.

In men, treatment options include two drugs that work to stop further thinning and maintain the hair you have. One drug is finasteride, an oral medication that targets testosterone. The other is minoxidil, a topical solution that helps increase the anagen phase of the hair cycle. Usually you have to use these drugs for about 12 months, to fully assess the benefits. (And note that only a small portion of men will see new hair growth.)

In women, treatment will often depend on whether there is excessive secretion of androgen; if so, drugs such as spironolactone, cyproterone acetate and Diane contraceptive pills may be prescribed. Minoxidil has also been used with some success for women.

Finasteride is not usually prescribed for women, especially those in child-bearing years, because it has an impact on testosterone production and there are concerns about the feminization of male fetuses.

Other ways to deal with hair loss include cosmetic aids, such as wigs and hairpieces. And many men, young and old alike, simply embrace the sleek look, cropping their remaining hair close to the scalp or shaving completely.

There is also the option of hair transplants, in which hair is taken from unaffected areas of the scalp and transplanted to affected areas. Hair transplants can be expensive, however, and can have temporary side effects such as facial or scalp swelling.

Twenty years ago, there were few treatments for hair loss, but today there are many ways to help deal with the problem. Don't be shy about talking to your doctor.

Dr. Marla Shapiro can be seen daily on Balance . . . television for living well on CTV. Questions about general health issues can be sent to her at: health@globeandmail.com (Please direct queries about personal health issues to your doctor.)