Hi Mars and Tessa,
The biggest cause of female hair loss is sex hormone imbalance, or fluctuations. An increase in androgens can be caused by menopase, or PCOS (polycystic ovarian syndrome), and this can cause hair loss.
Lori is quite correct, hypothyroidism, (underactive thyroid gland), can also contribute to hair thinning. Some women with PCOS have both an excessively high level of testosterone and an underactive thyroid. It's not uncommon to have multiple factors involved in female hair loss.
Heavy metals in your body, such as lead, mercury or cadmium can cause hair thinning. This is a possibility if you have lived near a mining site, or worked or lived with someone who works in an industry that uses metals.
One of the B vitamins called biotin is crucial for proper hair and nail growth. Some people, especially people who are blood type A, do not absorb B vitamins very well from food. In high doses, 5-8 grams daily (5000-8000 mg) of biotin can be effective in restoring head-hair growth, with no known side-effects. Some specialty shampoos feature biotin for this reason.
If you are gluten sensitive, that can also cause vitamin deficiencies, due to poor absorption of fats, and the fact that most vitamins are fat soluble.
A deficiency in essential fatty acids may result in dry, brittle hair, and possibly hair thinning or loss. The omega-3 fatty acids are especially important, such as those found in fish oil.
Also, a deficiency in manganese may contribute to hair loss. Roni's post covered a lot of this information.
Most of this information can be found in this website, if you want to read more detail about it:
http://www.ovarian-cysts-pcos.com/female-hair-loss.html
I hope this helps.
Tex (Wayne)