I recommend you have your thyroid checked -thinning here is one of the most common symptoms. AS well as TSH, make sure your doctor tests for both Free T3 and Free T4 (the "Free"s are the bioavailable hormones and not the "Totals"). Also, please have them test for antibodies: TPO and anti-thryroglobulin.
Few doctors will run all these tests as they're trained to look for organ failure and not autoimmune conditions. IF it is autoimmune (positive antibodies), then the doc should Rx replacement thyroid hormone - ideally a natural one like Armour.
This is important advice for anyone with thinning hair that extends beyone 6 mos post-partum as pregnancy and childbirth are one of the most common triggers for autoimmune thyroid disease which is one of the most common UNDIAGNOSED conditions of women. This is something we mamas often miss as we tend to attribute alot of issues to normal aging, in particularly fatigue. I had thought that was just what happened to us when we hit 40, had young kids and were working fulltime, despite regular exercise, a good diet and proper sleep.
If I had only known that it was my thyroid, I would have saved myself alot of suffering - low thyroid also contributes to chronic pain, sleep difficulties, GI issues, etc. Also, low thyroid contributes to problems with breastfeeding and just so many other issues - high cholesterol, predisposition to infection, other hormonal imbalances, etc. I can guarantee no one will do well during perimenopause or worse yet menopause if they don't deal with this potential issue.
And, most importantly if your doc won't run the recommended tests, please find another doctor. A good resource for finding a good doctor is the Texas Thyroid Yahoo group:
http://health.groups.yahoo.com/group/Texas_Thyroid_Groups/
This group is strictly moderated so that it remains a safe place for folks to share information and ask about doctors. So, please include a note that M. B from Mamasource recommended the group to you and you'll get added right away. There's oodles of info shared by that group and I learn alot from these other ladies.