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Have you been noticing the part on your scalp getting wider? Are you constantly pulling your hair out of the shower drain? Seeing more hair on your brush than on your head? Feeling less and less hair in your ponytail? Hair loss doesn't only affect men. By age 70, close to 40 percent of women experience female-pattern hair loss.

So what exactly happens?

The scalp has about 100,000 hairs-90 percent are growing and about 100 hairs are shed per day. The problem: When normal hair growth shuts down, more shedding occurs (over 300 hairs per day) and there are not enough new hairs growing to recover that loss.

Here's how to tell if you are losing your hair: Grasp a handful of hair from the base of your scalp and continue tugging gently to the end of your hair. If more than six hairs come out easily, that's a positive pull test suggesting active hair loss.

Here are the top 5 causes:

Hormones Aside from the post-partum temporary hair loss that over 50 percent of women experience, there are some other hormonal problems.

Close to 40 percent of women who experience hair loss have higher levels of male hormones, or androgens, and should have these levels checked by their doctor. Abnormal levels may also cause excess hair on the face, acne, and menstrual irregularities.

Low levels of circulating thyroid hormone can cause both scalp and eyebrow hair loss. If your thyroid levels are low, you may also experience fatigue, constipation, and an inability to tolerate the cold.

StressWhen you are stressed you may want to pull your hair out! But believe it or not, if you have experienced a major physical or emotional stress, your hair may actually fall out up to 3 months later. The trigger-anything from a major surgery or pneumonia, to a traumatic loss or personal strife. Rapid weight loss is also considered a stress on the body, so don't be surprised if you notice your hair falling out after a crash diet.

This type of hair loss is temporary once you recover from the stressful event. Within six months of recovery your hair should regrow and keep in mind, persistent stress on yourself may mean persistent stress on your hair.

MedicationsAside from chemotherapy, more than 5 percent of women may have hair loss from certain blood pressure medications, anti-seizure medications, blood thinners, or medications that have a high content of vitamin A. Birth control pills can also cause hair loss because they effect the hormonal balance in the body.

Anemia and nutritional deficiency Low blood counts mean low hair counts. If your body is lacking in iron, folic acid, or other essential vitamins and minerals, your hair will suffer. Have your doctor check your blood to evaluate for normal levels and prescribe supplements required.

Infection Fungal or bacterial infections usually result in patchy hair loss. Check the site for redness, crusting, or flaking. Your doctor may perform a biopsy to diagnose the problem and treat it with topical or oral antibiotics.

There are also some rarer causes of hair loss due to autoimmune diseases, such as alopecia areata affecting up to 5 percent of women, and a form of lupus which effects the skin. These disorders are diagnosed by biopsy and although there is no cure, they can be treated with steroids and certain medications which interact with the immune system.

So what if you or your doctor cannot find a cause? Are there treatments?

Minoxidil has been 30-40 percent effective for women with no underlying cause for their hair loss. It works best when the problem is less than 5 years old and hair loss is most prominent on the top of the head. Some doctors believe that higher doses of minoxidil may have even better effects, though the exact mechanism of how the drug stimulates hair growth is unknown. However, hair loss recurs once treatment is stopped and improvement generally plateaus after about a year.

The latest treatment: Hair transplantation - where healthy follicles from other parts of the scalp are transplanted to areas of baldness. This involves careful evaluation by a doctor who specializes in this kind of procedure.

Can you prevent hair loss?

If you are worried about your hair getting thin or falling out, try taking a multivitamin to prevent nutritional deficiencies, and take biotin supplements - studies have shown that biotin strengthens existing hair and stimulates hair growth.

What's more, do not underestimate the power of stress; try stress reduction techniques to avoid hair loss-a healthy head of hair needs a healthy state of mind.

Dr. Tania Mucci is a senior internal medicine resident at Mount Sinai Hospital in New York City. She graduated from Jefferson Medical College in Philadelphia and specializes in the latest advances in primary care and general health.