Did you change color or perm it recently? Has there been a change in diet? A change in physical activity? Have you travels anywhere recently? Are their other symptoms indicative of illness? And how much hair are we talking? Is there a history of pattern baldness in the family? All of these have a bearing on your question. I'm going to tell a story of how seeming unrelated things can affect the body with unusual results. Way back when I was a little boy (we are talking Waaaaaaaaay back) We went up to the mountains. My mother a drinker of water (she is always drinking water) fell in love with the natural spring water - so much so she insisted we load up a lot of jugs of water into the car. While we were there over night she drank and drank and drank swearing this was the best darn water she ever drank. Going back home, down the mountain stopping far too often for her to dump off excess water she had taken on. Also her lips turned a slight bluish color. By the time we reached the valley floor, we had to take her to the hospital. Come to find out that the water she had been drinking was rich enough in certain minerals to effectively strip out what little iron she had left - she had been boarder line anemic due to iron deficiency. The change in elevation mixed with being pushed over the threshold of iron deficiency lead to the blue lips - lack of oxygen. After being stabilized and even though iron was being supplemented she went through a longish period of losing hair - usually in clumps. For the rest of her life her hair did not return as full as it had been - - - - - In her case she was already at the threshold of iron deficiency - there were no 'clues' as how close she was. A couple of minor changes, being at a higher elevation and drinking water that was taking out iron lead to her having moderate to severe symptoms. Iron loss leads to hair loss - Also it is only one of many possible "issues" that can be present. If this is problematic for you or if this is a recent development you should consult with a doctor. Other issues which can be serious can also cause unexplained hair loss. Maybe not to the extreme of Cyanosis (oxygen loss), but it could be indicative of boarder line long term illness that if caught now will not be as bad if you wait for other symptoms.

Can be a lot of things. Have your thyroid tested for one. In fact just go to the doctor and ask him to do a complete blood work up. Unless you are genetically prone to baldness, hair loss can be a sign of something more serious going on.

It depends on the amount. Your body sheds and replaces hair constantly. It is normal to have some in the brush or some in the shower. It's a natural process. If your hair loss is significant (handfulls) it could be a nutritional problem or one tied to an illness. It can also be caused by chemicals used to change the hair (perm/dye/bleach). Eat better, include gelatin in your diet(it is good for hair, skin and nails.) If you are sick, get the proper medication.... (some medication can cause an imbalance which may result in some hair loss but it is usually a temporary thing). If you eat well and this happens, and you don't feel sick, and haven't done anything to your hair, get your doctor to do a work up. It can be a sign of some underlying medical concern. You might want to take multi-vitamins to supplement your nutritional intake.

I would worry more if it were my teeth coming out when I brushed them!

Well you loose hair everyday, but it depends how much your loosing. Is your hair dyed? Cos when my hair was dyed when i brushed it and washed it, it tended to come out in clumps and iv had to stop dying my hair.