What is causing diffuse hair loss and severe nape hair loss? (Photo)

There are a few causes of hair loss at the nape. One can only really diagnose hair loss with a careful scalp examination and review of the patient's story. Please see a dermatologist.

Female hair loss is a difficult analysis and requires many tests, not just thyroid testing. I simply can not write a textbook on the subject .  
The first step in evaluating hair loss in women, after a detailed history and physical exam, is to rule out any underlying medical causes of hair loss which can be treated. If clinically appropriate, the following disease processes should be considered: anemia, thyroid disease, connective tissue disease, gynecological conditions and emotional stress. Furthermore, over 50% of women going through change of life hormone fluctuations (menopause) experience significant hair loss. It is also important to review the use of medications that may cause hair loss, such as (but not limited to) oral contraceptives, beta-blockers, Vitamin A, thyroid drugs, coumadin, and prednisone. The following laboratory tests have been recommended to rule out the aforementioned medical conditions:
Some common blood tests for female hair loss:
Sex Hormone testsSHBG (Sex Hormone Binding Globulin) used to test status of male hormonesEstradiol is a sex hormoneFSH (Follicle Stimulating Hormone) *not hair follicle but follicle in the ovary*LH (Luteinizing Hormone) is a sex hormoneFree TestosteroneTotal TestosteroneANA (Anti Nuclear Antibody) used to test for Lupus or other autoimmune diseasesTSH (Thyroid Stimulating Hormone) used to test for hyper or hypo-thyroid diseaseTest Iron statusTIBC (Total Iron Binding Capacity)FerritinIron