Do you use hair dyes? They are known to cause hair thinning in addition to causing a number of health problems. Commercial, chemical shampoos and conditioners, nutritional deficiencies and/or pharmaceuticals can also cause hair loss\thinning.
My hair went from being course to very thin and brittle after taking prescribed medications... it could have been that in combination with the shampoos and conditioners I used. I am just now starting to see the trend stabilize after giving up the scripts about 1.5 years ago.