Standard advice is to have your doctor check your thyroid function. That was one of the first warning signs for me - slow growing hair and shedding more than usual. Didn't look like I was missing any, but I noticed. Now, I will notice that if my dosage of replacement hormone needs to be tweaked. Shampoo shouldn't cause more or less shedding, but it might impact how full your hair looks and whether you get any breaking off.