Stress is another big contributor to hair loss - you may notice that during particularly stressful times in your life, your hair tends to fall out more easily. The good news is that once the stressful event has passed, hair growth should improve.
An underactive or overactive thyroid can also lead to hair loss, but this tends to improve once the disorder has been treated and your thyroid hormone levels have returned to normal. However thyroid hormone replacement itself may also cause hair loss.
There is also an autoimmune disorder known as alopecia areata which leads to discrete patches of hair loss on the scalp. This needs to be treated by a dermatologist. Some nutrient deficiencies can also contribute to changes in the hair and nails, including zinc and selenium.
Finally, a number of medications can cause hair loss, including blood thinners, hormones, some antibiotics, some medications for high cholesterol, and certain meds for high blood pressure.
There are currently no definitive ways to prevent hair loss caused by aging, family genetics, or alopecia areata. There are certain medications that can stimulate hair growth, however, including spironolactone and minoxidi. Finasteride is another medication used for hair loss, but it is only indicated for men.
And of course, if stress is contributing to your hair loss, then consider starting a daily stress reduction practice like meditation or guided imagery. Don't forget to eat a healthy diet, loaded with fruits, vegetables, whole grains, nuts and seeds, to help prevent any nutrient deficiencies. A daily multivitamin/multimineral tablet may be protective, but talk with your doctor first if you have any medical issues for which vitamin or mineral supplements may be contraindicated.