Wayne-
I've lost my hair more often than I care to recall. First, when I was 10, I lost about 7/8ths of my hair when I was diabetic and hadn't been diagnosed. It grew back during junior high.
The next time I lost it, I was taking a new medication that had alopecia as a side effect. At that point I only lost in 2 areas on my head, basically my hair was making myself a widow's peak. That hair grew in, giving me fantastic evil-looking wings. (I was not amused.) This seems to be where I lose hair based on a medication, since it's happened since then as well.
I'm currently losing my hair but I'm not going to see anybody about it- it's stress. All the docs I ever saw said, it's stress, but if you want to prove it to me (the doc), collect your hairs in a bag, because everyone loses hair on a daily basis, and that's probably what is going on. (They didn't seem to understand I was telling them I was losing MORE than normal...) So this time, I'm not taking any medication or anything, but I've been under tons of extraordinary stress over the past few months (law school, job, and a husband I don't see all combined), so it makes sense it's falling out. My hairdresser told me once (she loses her hair a lot too) that it can take 4-6 months for your head to get the message about your stress. I look at it with a "what happened 4-6 months ago" kind of analysis, because I felt like I lost my hair a few months after we got marries, too.
This is a long post, and I'm sorry, but I feel like I'm an expert by now! I've had all sorts of reasons, but now I just look at my hair loss as a way to gage what's going on in my life! I really hope that helps. I'd always go to a doc first, then go from there.