here are the most common reasons:

diet change
stress
medication

If you changed your diet in any way, you are likely missing out on some sort of vitamin. Figure out what you changed…and what you are most likely missing. For most women, it is protein.

If you are stressed, your hair will naturally follow suit. You need to get out of the stressful environment, or find a way to relax yourself. Don’t worry…when the stressful situation is over, your hair should come back full swing with the rest of you.

Medication (even something like asprin) will cause your hair to thin out. There is not much you can do about it. You can try talking to your doctor about switching your medication.

A more uncommon cause of hair thinning is genes. For some women (very few) hair loss is hereditary! It is a sad thing, and nothing can be done about it.

There are products like Roggaine and Nioxin that help to preserve the hair that you do have. It will not make your hair grow back, but it will strengthen the hair that you do have. The problem with these products is that they only work when you use them, and they are expensive.

Try eating more avacado. They are fantastic for your hairs health and shine!

Ps. if the thinning problem is actually just a ‘my hair has always been thin’ problem…there is not too much you can do about that either…there are some fantastic products out there designed to give your hair some body. talk to your hair stylist about some products that would help your hair type look the most full and most natural.

i like paul mitchells extra body line.
References :
i am a hairstylist.