What are the top reasons for women's hair loss?

Hypothyroidism: Low thyroid or vitamin, like biotin deficiency.

Multiple: Difficult to narrow down as there are entire books written on this topic. 1st is to determine if it is scarring or non-scarring. If just thinning and non-scarring, is hair slowly getting thinner (female pattern) or falling out in clumps (telogen effluvium). Also, alopecia areata is non scarring and can be coin shaped area on scalp to full body. I usually check thyroid and iron stores too.

Pregnancy, diet: Post-partum - after childbirth (weeks to months), poor diet (restricting calories, limiting protein), major physical stress (surgery, illness, anesthesia) or psychological stress, thyroid disease, rheumatologic disease -like lupus, dermatological disease - like psoriasis, alopecia areata. May also be caused by pulling hair back too tightly in ponytail/braids.