Over the past year, I've noticed my hair must be thinner because a ponytail holder wrapped around twice slides out of my mid-back length hair (whereas it used to stay in).

I have already tested for thyroid problems, and none came up.  My dermatologist "said" my hair count was "within normal range."  I don't know what the problem is-- any ideas?

Stress and/or hormones can cause thinning.  My hair has about 1/2 the thickness it had when I was in my 20's, but it is still considered normal.

Some people find that taking MSM, biotin and silica help reduce the shedding.  Silica had most effect for me.

Of course, being gentle with you hair should always be at the top of the list.