I have been dealing with this for over 2 years and it has gotten quite pronounced. I have never gotten an answer beyond, it could be thyroid, but, that has been regulated w medicine for 1.5 years, could be low ferritin & iron which supplements were not helping, and had an iron infusion. I gave up gluten for a year to see if it helps.
Nothing has.

OP, I experienced significant hairloss at 21 with what turned out to be a vitamin A overdose from my multivitamin suplement. Weird, right? It stopped and grew back once I quit taking it. (It was a Schiff vegetarian multvitamin and this was in 1995). I was likewise ata total loss and brought in my supplements to the doc as a last ditch effort to figure it out.

I do have other hypothyroid symptoms that all started to appear around the same time as my hair started thinning-- tiredness, slight weight gain, stopped producing earwax (rare symptom that my ENT pointed out when I started getting ear infections), and heavy periods. My TSH was 4.3 a couple months ago when the hair loss started. My T4 was right in the middle of the range. Thing is, before and during my pregnancies my TSH was always nice and low so this seems like a big jump even if it's not extreme. I've been thinking about seeing an endocrinologist but was thinking I need a referral.

Was wondering about vitamin D. My levels haven't been tested.

My hair was getting quite thin. It may be a coincidence, but I was diagnosed with a vitamin D deficiency, and after I started taking prescription supplements, my hair thickened up considerably.

Have your doctor check your thyroid antibodies. Do you have other hypothyroid symptoms? 3:45 is borderline treatable if you are symptomatic. I'd go see an endocrinologist, regardless.

http://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0001393/

http://www.webmd.com/skin-problems-and-treatments/hair-loss/features/women-hair-loss-causes?page=2

I feel like there have been quite a few women posting about hair loss on this forum, myself included. For me, it seemed to start all of a sudden around August last year and it is still ongoing. My TSH is 3.45 (and the doctor doesn't want to treat it), my iron stores were a little low (but supplements have not helped), and I tested negative for lupus (twice) and whatever hormones relevant to PCOS. My hair is still thinning and, given the rate at which it has been falling, the fact it grows back (I have tons of little stubbies), and the "a little all over" pattern, I just can't imagine this is female pattern baldness. I'm getting frustrated that I won't figure it out until I'm bald. Any inspiration?? Could it have something to do with gluten? (Seems far-fetched, but I'm getting desperate.)